Williams also ended his successful loan-deal with Oxford United on a sour note with a red card for an off the ball skirmish that handed the advantage to Northampton Town on Saturday.
Williams put the League Two side ahead on six minutes but was dismissed just on 20 minutes for kicking out at The Cobblers midfielder Richard Ravenhill.
Northampton went on to win 3-1 and escape relegation.
Williams’ strike capped off a strong season for the Perth-born flyer, who notched up eight goals in 42 appearances for Oxford after arriving on loan from Fulham last August.
Before his final match for the Yellows - and Fulham's heartbreaking loss to Stoke - the youth international talked up his development over the past season.
"I go back to Fulham not only a better player but stronger mentally", Williams told the Oxford Mail.
"I think I could do a job for Fulham and, fingers crossed, it’s in the Premier League.
"I’m just going to try and play at the highest level possible."
Oxford manager Gary Waddock was less than impressed with Williams’ moment of ill-discipline which turned the game on its head.
"That 90 minutes probably summed Oxford’s season," Waddock told the Northampton Chronicle. "We were very good for parts of the game but then faded away.
"The red card was the turning point. Ryan had started the game very well, especially with the goal. It was delayed reaction from him but you can’t act like he did and get away with it.
"But we conceded poor goals, which were down to errors and never got back in it after that."
Oxford finished one spot off the play-offs.