Obama watched from the players' box along with Williams' sister Venus as the fourth seed took just 61 minutes to see off former world number one Jankovic 6-3 6-1.
"It was great seeing Michelle there. It just made me want to play better and I felt even better. We're here for our country and to play well. I loved her dress, which is always nice. She is always looking good," Williams said after the match.
With the usual Wimbledon requirement for players to wear white not in place for the Olympics, Williams herself took to the court in a red, white and navy blue dress with a red headband, while Jankovic also wore red.
"It's so different than playing in all white and I was thinking 'I wouldn't like it', but I like it," she said, although then adding that she would "absolutely not" like to see coloured clothing introduced at Wimbledon.
With the first set comfortably under her belt, five-time Wimbledon winner Williams took an early break in the second set when Jankovic hit the ball into the Olympic-ring adorned net.
The American quickly broke again to storm to a 3-0 lead, then denying her opponent a single point in the fourth game. Jankovic managed to hold her serve one more time, but appeared increasingly frustrated with herself, wrongly challenging two line judge's calls in a row.
Serving to stay in the match at 15-40, the 27-year-old was given a momentary reprieve when her opponent powered a backhand into the net.
The American, who has two doubles gold medals with Venus but is still seeking a single gold, let out a squeal of annoyance but she didn't have to wait long to secure her victory as Jankovic hit the next shot wide to gift her the match.