He is the son of a Puerto Rican father and a Jewish mother whose family have roots in Romania and Russia.

When that partnership ended, Agosto moved from Chicago to Michigan in 1998 in order to train under Igor Shpilband.

In the 1999–2000 season, they won a pair of medals on the ISU Junior Grand Prix series and finished 4th at the JGP Final.

In 2001–2002, Belbin/Agosto made their senior Grand Prix debut and won another silver medal at the U.S. Championships, which would have qualified them for the 2002 Winter Olympics if Belbin had been an American citizen.

At Nationals in 2005, the last year of the 6.0 system, they received straight perfect sixes for presentation in their free dance.

Their silver medal combined with the placement of the other American team earned the United States three spots to the Olympics in ice dance, the first time this had happened since 1984.

By a special act of Congress that passed on December 28, 2005, which President Bush signed on New Year's Eve, 2005, Belbin became a naturalized citizen of the US, making her able to compete for the United States at the 2006 Winter Olympics.

Belbin/Agosto began the 2006–2007 season with a free dance called That's Entertainment but arrived at Nationals with a new program to the music of Amelie.

[9] In addition to teaching different technique,[8] Linichuk advised Belbin to gain 10 pounds and develop some muscles in order to skate faster and more fluidly.

[9] This also gave Belbin more core strength to hold her positions better, thus making lifts easier for Agosto.

They withdrew from the 2008–2009 ISU Grand Prix Final after the original dance due to a back injury to Agosto.

[5] Agosto also worked as a voice actor for a video game in 2006, and expressed an interest in continuing in the field.