Early in his time at the University of Illinois, Andreessen started working in a research lab at the university's physics research facility.

True: biographical records show that early in his time as a University of Illinois student (roughly his first year or two), Andreessen began working in a university research lab that can fairly be described as a physics research facility. Mostly true: he did start research-lab work early, but the facility was not a physics facility (e.g. a supercomputing centre such as NCSA, or another non-physics lab), or the timing was a little later than 'one of the first things'. Mostly false: he worked at a university research lab only late in his studies, well after starting. False: records show he did not work at any University of Illinois research lab as a student.

Set before the research began.

1. E1 Marc Andreessen | Biography | Research Starters | EBSCOhost EBSCO Research Starters · 2013

   “In 1989, Andreessen entered the University of Illinois at Urbana-Champaign. In 1992, he got a part-time job working in the NCSA, a government-funded research lab at the university.”

2. E2 Marc Andreesen and Eric Bina Lemelson-MIT Program · 1996

   “In 1992, while Andreesen was an undergraduate student and part-time assistant at the National Center for Supercomputing Applications (NCSA) at the University of Illinois, the World Wide Web had just begun to take off.”

3. E3 Andreessen Horowitz Part I | Acquired Acquired (podcast) · 27 July 2021

   “Marc—when he gets to Illinois—he immediately starts doing work-study at the NCSA and while he's there as you mentioned, he's not yet Marc Andreessen.”

All three sources place Andreessen's University of Illinois lab work at NCSA (E1, E2, E3), which E2 names as the National Center for Supercomputing Applications; none describes it as a physics research facility. The sources disagree on when he started. E1 says he entered in 1989 and took the part-time NCSA job in 1992, which is well into his studies. E3, a podcast, says he started work-study at NCSA as soon as he arrived, and E2 confirms only that he was working there in 1992. Under the criteria, an early start would make the claim mostly true and a late start would make it mostly false; because the evidence conflicts on timing, the verdict is mixed.