\Watson failed to open his account in the first 10 balls of the innings, but there was no looking back once he got into the groove.
Time seemed to have robbed Yuvraj of his touch and the 36-year-old had to scratch around for 65 runs from six innings, while going wicketless with the ball.
For someone who wielded a willow that appeared to have sweet-spots wherever the ball made contact, Yuvraj struggled to find the middle of the bat against modest bowlers in less demanding circumstances.
With Chennai needing 21 runs in 20 balls, the duo kept on playing freely and overhauled the target easily to become the only team to beat a side four times in a single season.
Chennai Super Kings won the toss and chose to field.
The New Zealander was disappointed that the team could not win the all important final after posting 178 for six.
Williamson was the top-scorer in the tournament, with 735 runs at an impressive average of 52.50 and strike rate of 142.44.More news: Meghan Markle's half-sister rushed to hospital after auto crash
He was supported by Yusuf Pathan who hit an unbeaten 45 off 25 balls, including four fours and two sixes.
While Du Plessis couldn't emulate his previous performance but it was his opening partner Watson who stuck around for a while.
The Australian along with Suresh Raina (32) forged a 117-run match-winning partnership to put Chennai ahead of their opponents.
The big-hitting Aussie allrounder plundered 27 runs off one over of paceman Sandeep Sharma.
At the auction, CSK were criticised for picking up nine players in their 30s including Shane Watson and Ambati Rayudu, who eventually were instrumental in the team's triumph. CSK has won over SRH by 8 wickets.
"It is the fitness that really matters more than the age aspect", said the former India captain, 36.
Captain Kane Williamson earlier became only the fifth player to make 700 runs in an IPL season, the New Zealand worldwide scoring 47 in the Sunrisers' total of 178-6. "If I push Watson to stop a single, there is a very good chance that he´ll burst his hamstring and won´t be available for the next game".
The winners of the final were guaranteed a minimum $4 million in prize money.