The 27-year-old Swiss star, a one-time world number four who is now ranked 487, took down France's Chloe Paquet 6-3, 6-1 on Ken Rosewall Arena in Sydney, winning 10 of the last 11 games.
While she tentatively returned to the court at low-key events in November after having a baby in April, it was her first win at tour-level since her third-round victory at the 2023 US Open.
Her teammate Dominic Stricker crashed 6-3, 7-5 to Ugo Humbert in their singles rubber to send the tie to a deciding mixed doubles.
Bencic fronted up again alongside Stricker to down Edouard Roger-Vasselin and Elixane Lechemia 6-1, 7-6 (7/4) to earn Switzerland the win.
She said it was encouraging to beat Paquet, who broke into the top 100 for the first time last year.
"I'm really excited to be back, of course," said Bencic, who won Olympic gold at the 2021 Tokyo Games and has eight career WTA titles to her name.
"I had a little bit of nerves at the start and a little bit of rust, so I'm happy I kind of accepted that and tried to fight through it.
"And after, I felt like it went smoother, I became a little bit more calm, and kind of was more able to focus on my tennis."
The 18-nation mixed-team United Cup is being played in Perth and Sydney.
Each team features three men and three women, with ties comprising one men's and one women's singles and an often decisive mixed-doubles clash.
The Alex de Minaur-led Australia meet Argentina in Sydney later.
Canada and Croatia kick off the action in Perth with Greece's Stefanos Tsitsipas and Maria Sakkari headlining the evening action against Spain.
The tournament began on Friday with Spain crashing 2-1 to Elena Rybakina's Kazakhstan, while China swept past Brazil 3-0.
