The former Great Britain and Wigan rugby league captain played 65 minutes of Saracens' Guinness A League game against Harlequins on Monday night - his first competitive action for almost two years.
A depressing injury catalogue - which included knee, toe and back problems - meant a frustrating, prolonged spell on the sidelines after 31-year-old Farrell switched codes 18 months ago.
But a try-scoring contribution and the fact he emerged unscathed from Monday's action means a probable appearance off the bench when Saracens tackle Newcastle at Vicarage Road on Sunday.
"Everything is good," said Farrell. "The bit of stiffness I had the following morning has gone, and when I was sat at home yesterday I couldn't help but smile to myself at finally getting that post-match feeling back into my body.
"I have also had the time to look back at the match.
"I must say that I realise now how much I must have had going on in my mind during the game, because I didn't remember that much about it, but watching the video has brought a lot back.
"Clearly, there are some things I need to address. Defending the channel off the scrum, clearing out at rucks and mauling are some of those, but the more I play the quicker I will adapt and become a better rugby union player," Farrell told Saracens' official website.
"One of the best things I have enjoyed (this week) is being back in a match squad again.
"I have played a game, come through it okay and now I am fighting for a place with the rest of the squad.
"That is something I have missed massively, and it is hard to describe just how good it feels to being part of that again."
Saracens have yet to record a Premiership victory this season, but they will start as favourites against a Newcastle side again without the services of injured fly-half Jonny Wilkinson.