He has put pen to paper on a deal that takes him to the summer of 2025, having joined Fulham from Perth Glory a year ago.
The 20-year-old has impressed for Fulham’s under-21s, but has not yet made the jump up to the senior team, as he went out on loan to Airdrieonians during the second half of the season.
After renewing, Donnell told fulhamfc.com: “I’m buzzing to sign a new deal and hopefully I can kick on now.
“Last year was a big year for me. I enjoyed spending the first six months of the season here, but then I felt it was right to go out on loan and get some First Team minutes, which I really enjoyed and I learned a lot.
“This year, I want to add numbers to my game and influence the team; whether that’s defensively or going forward with goals and assists.”