Max Mullen

Partner, Rebel Fund; Co-Founder, Instacart at Rebel Fund

Reviewed Updated May 1, 2026

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Location San Francisco, California
Check Size $25K–$500K (sweet spot ~$200K per Signal NFX)
Last Verified Investment ChampAI (Seed ($9M)) — May 2026
Social @Max LinkedIn
Stage Focus
pre-seed seed series-a
Sector Focus
b2b-saas marketplaces fintech ai consumer enterprise devtools

Background

Max Mullen is an American entrepreneur and angel investor, best known as a co-founder of Instacart, the online grocery-delivery company he started in 2012 with Apoorva Mehta and Brandon Leonardo 1 2. Instacart went through Y Combinator’s Summer 2012 batch and went public on the Nasdaq (ticker: CART) in September 2023 2 3.

In the early years of Instacart, Mullen wore many hats — product, design, customer support, payroll, and office operations — before narrowing his focus to culture and company building 4. He stayed with the company for roughly fourteen years, through its 2023 IPO and beyond 2 5. In April 2026, Mullen announced he was stepping back from his day-to-day role at Instacart to focus on early-stage investing, transitioning into an advisory capacity at the company 5.

He runs Workshop, a founder community in San Francisco, and is a Partner at Rebel Fund, the YC-alumni-only seed syndicate founded by Jared Heyman in 2019 4 6 7.

Stated Thesis

Mullen has not published a formal investing thesis, but in his April 2026 statement announcing his step-back from Instacart he framed his investing motivation directly:

“Building Instacart has been the privilege of a lifetime and I’m incredibly energised to use my experience to help the next generation of founders — people with the courage to work on the hardest problems — as an early stage investor.” 5

His self-reported scope on Signal NFX is pre-seed, seed, and Series A, with sector interests in e-commerce, AI, marketplaces, enterprise, and consumer internet, and a check-size range of $25K–$500K (sweet spot ~$200K) 8.

Inferred Thesis

Counted portfolio: 52 verified investments on Signal NFX 8; Mullen himself claims “100+” startups backed 4. The list below is therefore directional, not exhaustive.

  • Stage: predominantly seed, with selective pre-seed and Series A follow-on. Signal NFX’s verified entries skew to seed rounds in the $3M–$12M range, with occasional pre-seed checks and growth follow-ons (e.g., Stedi Series C, March 2026) 8.
  • Sectors: B2B SaaS, fintech, marketplaces, AI/devtools. Named portfolio companies on his personal site include Checkr, Deel, Lattice, Mercury, Newfront, Stord, Anomalo, Ashby, Middesk, Stedi, Superblocks, Verifiable, Span, Multiply, Frec, Savvy Wealth, Float Health, Honeydew, Gumloop, You.com, Rootly, Shef, Owner, Crosby, Champ, AtoB, Highstock, Nuvo, Omni, and Tangle 4. The center of gravity is B2B software for operators — payroll, compliance, EDI, brokerage, identity, code review — plus marketplaces (Owner, Shef, AtoB) and a smaller AI/devtools cluster (Gumloop, Crosby, You.com, Rootly).
  • Instacart-alumni concentration. Mullen explicitly highlights backing Instacart alumni — public examples include Anomalo, Cabal, Highstock, Pequity, and Rootly 9.
  • YC alignment via Rebel Fund. As a Rebel partner, Mullen participates in a fund whose portfolio is 100% Y Combinator alumni 6 7.
  • Co-investors skew operator-angel and large-fund. Signal NFX names Jack Altman (Benchmark), Andrew Chen (a16z), Tobias Lütke (Shopify CEO), Lee Fixel (Addition), Guillermo Rauch (Vercel), and Charlie Songhurst as frequent co-investors 8.
  • Active-investor signal: strong. Verified 2026 checks include ChampAI (Seed, $9M, May 2026), Stedi (Series C, $50M, March 2026), Multiply (Seed, $10M, March 2026), and Unusual AI (Seed, $4M, January 2026) 8. The cadence is consistent with a full-time angel rather than an occasional operator-investor — and is poised to grow further now that he has stepped back from Instacart 5.

Caveat (sample size): Mullen claims 100+ angel investments 4; Signal NFX has verified 52 8. Many private checks are not publicly disclosed. The patterns above are based only on the verified subset.

Portfolio

Selected verified investments. This list is illustrative; Mullen states he has backed 100+ companies 4.

Company Year Stage Sector Source
ChampAI 2026 Seed ($9M) AI 8
Stedi 2026 Series C ($50M) Fintech / EDI infrastructure 8
Multiply 2026 Seed ($10M) Fintech 8
Unusual AI 2026 Seed ($4M) AI 8
Sweatpals 2025 Seed ($12M) Consumer 8
Hero Assistant 2025 Seed ($4M) AI / productivity 8
Palabra.ai 2025 Pre-Seed ($8M) AI / voice 8
Willow Voice 2025 Seed ($4M) AI / voice 8
~unknown Deel — Multiple B2B / global payroll
~unknown Mercury — Multiple Fintech / banking
~unknown Lattice — Multiple B2B HR
~unknown Checkr — Multiple B2B / background checks
~unknown Newfront — Multiple Insurtech
~unknown Stord — Multiple Logistics
~unknown Anomalo — Multiple Data / Instacart alumni
~unknown Ashby — Multiple B2B / recruiting
~unknown Middesk — Multiple B2B / KYB
~unknown Owner — Multiple Restaurant SaaS / marketplace
~unknown Superblocks — Multiple Devtools
~unknown You.com — Multiple AI search
~unknown Rootly — Multiple Devtools / Instacart alumni

Round-specific years for the lower portion of the table are not consistently published on Mullen’s personal site; only the company names are listed 4. Investments are attributed to him as an angel; many overlap with Rebel Fund deal flow but are not individually broken out in public filings.

In Their Own Words

On his shift from operator to full-time investor (announcing his April 2026 Instacart step-back):

“With the strength of our business, a clear vision, and a world class leadership team in place, I’m moving into an advisory role at Instacart with complete confidence in the company’s future.” 5

“Building Instacart has been the privilege of a lifetime and I’m incredibly energised to use my experience to help the next generation of founders — people with the courage to work on the hardest problems — as an early stage investor.” 5

On culture as a founder’s job (First Round Review podcast):

“Culture is the consciousness of the company, which includes its beliefs, its behaviors, and the norms.” 10

“Values are really what we stand for as a company and what we expect of each other.” 10

“Culture was a part of my job as a founder, even though we didn’t have our culture written down yet.” 10

On organizational hygiene:

“You have to really have little tolerance for politics. When it happens, you notice it, you have to jump on top of it.” 10

What Founders Say

No independently sourced founder testimonials about working with Max Mullen as an angel or Rebel Fund partner were found after dedicated searching. (Rebel Fund’s profile carries the same disclaimer — syndicate and angel checks rarely surface in public founder commentary, which tends to credit lead investors.)

Connections

  • Co-founder, Instacart (2012–2026, day-to-day; advisor 2026–) — built the company alongside Apoorva Mehta (CEO) and Brandon Leonardo; the company went public on Nasdaq (CART) in September 2023 1 2 3 5.
  • Partner, Rebel Fund — invests alongside fellow YC-alumni partners Jared Heyman (CrowdMed), Trip Adler (Scribd), Daniel Kan (Cruise), Steve Huffman (Reddit), Sebastian Mejia (Rappi), and Dan Siroker (Optimizely) 6 7.
  • Y Combinator alumnus (S12) — Instacart graduated from YC’s Summer 2012 batch 7.
  • Founder, Workshop — runs a San Francisco–based founder community 4.
  • Frequent angel co-investors (per Signal NFX): Jack Altman (Benchmark), Andrew Chen (a16z), Tobias Lütke (Shopify), Lee Fixel (Addition), Guillermo Rauch (Vercel), Charlie Songhurst 8.
  • Instacart-alumni network — actively backs former Instacart employees building new companies, including Anomalo, Cabal, Highstock, Pequity, and Rootly 9.

Sources

  1. “Instacart,” Wikipedia, accessed May 2026. https://en.wikipedia.org/wiki/Instacart↩↩

  2. “Instacart’s founder is worth a cool $1.3 billion after IPO—and today he cut all ties with the company,” Fortune, September 19, 2023, accessed May 2026. https://fortune.com/2023/09/19/instacart-cofounder-net-worth/↩↩↩↩

  3. “Max Mullen — Co-Founder @ Instacart,” Crunchbase person profile, accessed May 2026. https://www.crunchbase.com/person/max-mullen↩↩

  4. Max Mullen — personal website (maxmullen.com), accessed May 2026. https://www.maxmullen.com/↩↩↩↩↩↩↩↩

  5. “Instacart co-founder Max Mullen steps back from day to day role at grocery technology firm,” Retail Technology Innovation Hub, April 17, 2026, accessed May 2026. https://retailtechinnovationhub.com/home/2026/4/16/instacart-co-founder-max-mullen-steps-back-from-day-to-day-role-at-grocery-technology-firm↩↩↩↩↩↩↩

  6. Rebel Fund — homepage and partner roster, accessed May 2026. https://www.rebelfund.vc/↩↩↩

  7. “Rebel Fund: The Early Stage Founder’s Guide,” Superscout, accessed May 2026. https://superscout.co/investor/rebel-fund↩↩↩↩

  8. “Max Mullen’s Investing Profile — Angel,” Signal NFX, accessed May 2026. https://signal.nfx.com/investors/max-mullen↩↩↩↩↩↩↩↩↩↩↩↩↩↩↩

  9. “Max Mullen, Angel Investor,” AskForFunding, accessed May 2026. https://askforfunding.com/investor/max-mullen-angel-investor↩↩

  10. “Instacart co-founder Max Mullen gets tactical on crafting company values and intentionally building culture,” First Round Review (In-Depth podcast, Episode 42), accessed May 2026. https://review.firstround.com/podcast/instacart-co-founder-max-mullen-gets-tactical-on-crafting-company-values-and-intentionally-building-culture/↩↩↩↩