His investments have included Bullhorn (acquired by Vista Equity Partners), BzzAgent (acquired by Tesco), Maven Networks (acquired by Yahoo) and Taleo (taken public, later acquired by Oracle).

Simon confirmed his departure to Fortune, emailing over the following statement:

“Not saying much beyond things have gone great and are going great at GC (some examples include today’s Kayak IPO and the recent sales of Bullhorn and Quickplay, both of which I was fortunate enough to be involved with, along with numerous past and pending exits) and that gives me the opportunity to step away, keep the relationship and responsibilities and opportunities with the board seats with almost all of my portfolio companies (a number of which I co-founded and chair and which are generally doing exceptionally well) and free up the bulk of my time to pursue new, new things. Hope and trust the next 12 years will be as impactful and meaningful as the last 12 co-founding and helping build GC.”

Amicable sentiments, but sources suggest that the situation was a bit more complicated.

Simon co-founded the firm with three other venture capitalists: Joel Cutler, David Fialkow and David Orfao. When Orfao quietly cut back on some of his management responsibilities a year or two ago, it basically left just three co-founding decision-makers, with Cutler and Fialkow having known each other since childhood.

Soon Simon began finding himself on the short side of several decisions, and broader strategy moves like General Catalyst’s strong push into seed-stage Silicon Valley consumer deals (he felt there should be more focus on deep-tech opportunities closer to the firm’s Massachusetts roots).

“It was a combination of things, but leaving now lets John continue  to stay involved with his portfolio companies and find something new to do,” one source says. “He just turned 50 and has been with General Catalyst for a long time. Sometimes people just need to go their separate ways.”

No word yet on Simon’s future plans, except that he doesn’t seem to have finalized them yet. Do expect him to continue seed investing, which he already had been doing at General Catalyst.

I also haven’t heard if General Catalyst plans to promote anyone to assume Simon’s managerial responsibilities, although word is that managing director Hemant Taneja stepped into some of those roles when Orfao stepped back.

A General Catalyst spokeswoman did not return a call requesting comment.

Sign up for Dan’s daily email newsletter on deals and deal-makers: GetTermSheet.com