“Every match was incredible,” Benneteau told fedcup.com. “I am so proud of them. It was always my intention to put them together — and it was not hard to do.”
Earlier Sunday, French Open champion Halep won her second singles of the weekend, seeing off Garcia 6-7 (6/8), 6-3, 6-4 for a 2-1 lead in the semi-final.
Veteran Pauline Parmentier then beat a hobbling Irina-Camelia Begu 6-3, 2-6, 6-2 to pull France level.
In a tense deciding rubber, Halep, whose doubles ranking is in the 400s, committed a double fault to concede the break in the ninth game of the final set before Garcia held her nerve to serve out the victory.
France, who won the last of their two titles in 2003, will travel to Australia for the final.
Six-time champions Australia, whose most recent title was in 1974, reached a first final in 26 years by seeing off Belarus 3-2 in Brisbane in a tie also settled by a three-set deciding doubles rubber.
Halep gave Romania a lead on Saturday by seeing off Mladenovic 6-3, 6-1 before Garcia swept past Mihaela Buzarnescu 6-3, 6-3 to level the tie.
“I knew it would be more difficult than yesterday,” said Halep after registering a ninth successive singles win in a tournament that Romania have yet to win.
“It felt like playing at home, that was how I was able to fight to the end. I won it for the fans who came here.”
Halep’s win over Garcia, her sixth in seven meetings, took almost three hours with the crucial break in the final set coming in the eighth game when the French number one served up back-to-back double faults.
Australia made the championship match when Ashleigh Barty and Samantha Stosur battled past Victoria Azarenka and Aryna Sabalenka 7-5, 3-6, 6-2.
The tie was sent into the crucial doubles match after two-time Australian Open champion Azarenka crushed Stosur 6-1, 6-1 in the reverse singles, after Barty swept past Sabalenka 6-2, 6-2.
“I could not have picked two more perfect athletes to compete. Ash and Sam are superstars,” said captain Alicia Molik who will look to inspire a first title since 1974 for Australia in the final.
Barty called making the final “unbelievable” while Stosur said it was “super exciting”.
“We knew it was going to be a tough one and we just so happy with how we played,” added Stosur. “All the girls have worked so hard throughout the year and we really stick together.”