Williams and Frances Tiafoe were already eliminated after losing their opening two ties.
Watched by her daughter Alexis Olympia, Williams’ 6-1, 7-6 (7/2) win over the 97th-ranked Boulter followed victories over former top 10 player Belinda Bencic and Greece’s Maria Sakkari earlier in the week.
Currently ranked 16th, Williams needs just one more Grand Slam singles title to equal the record of Australian Margaret Court but hasn’t won a major since the Australian Open in 2017.
She gave birth to her daughter in late 2017 and was beaten in the Wimbledon and US Open finals last year.
It was at the latter that she had a major meltdown during her loss to Naomi Osaka.
With the Australian Open in Melbourne less than two weeks away, Williams dropped just one set in her singles matches, although at different times she seemed to be troubled by a right shoulder problem and had both ankles strapped.
Her form was patchy across the three matches, but Williams said the round robin format of the event was ideal preparation for Melbourne.
“It was definitely good to play three singles matches here, not to mention the mixed,” she said after her singles match.
Williams cruised through the first set against Boulter but found a more competitive opponent in the second.
Not for the first time this week, the frustration of the American grew as Boulter was able to match her for winners, with the Brit claiming an early break in the second game.
Williams immediately broke back, but then enjoyed a slice of good fortune when her forehand dropped over the net on a Boulter break point in the fourth game.
The set remained on serve until the tie-break, which Williams dominated.
In the men’s singles, the struggling Tiafoe led 5-3 in the first set but won just one more game in losing 7-6 (7/4), 6-0 to Cameron Norrie.
The Brits claimed the tie with a victory in the mixed doubles.