Mendez was born in Emilypolis to a Mexican father and a Paraguayan mother. Educated at home for most of his school years, Mendez did not attend university until much later in life, instead choosing to travel and work. He spent several years in Saydney, South America and Europe before returning in 1975. Overseas he had become attracted to moderate leftist politics and joined the Democratic Party of the United Islands from the party's early days, remaining a member until 2004. He became the Scoitan party's state President in 1984. The Democrats had little presence in Scoita and were written off electorally for most of their history. In 1987, however, they elected one Senator in Scoita, their first in that state, Norman Grosvenor. Mendez was the Democratic Party's lead campaigner in that election and gained a reputation as a shrewd political tactician and party organiser. In 1991 Mendez contested the seat of Cathedral for his party but was defeated. In 1995 he was elected to Grosvenor's seat in the Senate.
As a Senator, Mendez was one of his party's most visible members, due in part to his desire to have a high profile. Mendez became known as the "background Senator" because of his place in the chamber just behind the Leader (and therefore directly in the camera's view) and to his appearances behind Labour or Democratic politicians when they made public appearances in Scoita.
Mendez was re-elected at the second 1995 election, and again at the 1999 dissolution. His seat was not initially up for election in 2002, but after the third double dissolution in a decade was called, Mendez was re-elected, for the first time earning a quota of votes without flow-on from the Labour Party. In 2004, Mendez was personally opposed to the merger of the Liberal and Democratic parties, but joined the new Liberal Democratic Party after its formation.
Mendez's seat was not up for re-election at the 2005 election, and instead he campaigned heavily for the Liberal Democrats in Scoita. In the same year he aided the state party in its attempt to increase its seat totals in the Scoitan state legislature, which it did. Mendez was not offered a ministerial position when Zoe Parker replaced Campbell Rhodes as Prime Minister, but continued to sit on several important Senate committees and in December 2005 became Chairman of the Senate Education and Youth Affairs committee.
In late 2006, Mendez declared his intent to seek to become Scoita's next Governor, to be replaced by Vance Thorpe. He formally announced his candidacy on December 15. He resigned from the Senate on February 2, 2007, to campaign full time for the Liberal Democratic nomination. At a party conference in April, Mendez was nominated by the LDP to be its gubernatorial candidate.
Mendez's gubernatorial campaign impressed many commentators who suggested he might be the non-Conservative party's best chance in decades to capture the Governorship, which has been held by either a Conservative or a right-leaning Independent since 1979. A number of pundits and commentators have pointed to Mendez's "textbook" approach to campaigning, laconic and witty speaking style, not to mention questions around Conservative Mick Pearson's qualifications for the position, as reasons why Mendez is a real threat to the Conservative campaign.
Early exit polls on election day showed Mendez almost neck-and-neck with Pearson, with indications Conservatives were not turning out to the extent of Liberal Democrats. This indicated a trend towards Mendez in the general electorate. By the end of the vote count, Mendez had won 43.37% of the vote, compared to 40.6% for Pearson. Mendez is the first Liberal Democratic governor and the first leftist Governor in decades. He took office on September 19.