Thirty-eight years,
told in sequence

Fonseca completed a Bachelor of Science in economics with honours at the University of Louisiana at Lafayette, then known as the University of Southwestern Louisiana.

His first post on returning home was as a research officer at the Society for the Promotion of Education and Research, from 1989 to 1990, followed by a year as an economist in the Ministry of Economic Development from 1990.

He took an LL.B. with honours from The University of the West Indies in 1994, and his Certificate of Legal Education at the Norman Manley Law School in Jamaica in 1996 — the qualification that admits a Commonwealth Caribbean graduate to practise.

He served as Crown Counsel in the Office of the Director of Public Prosecutions from 1996 to 1997, prosecuting on behalf of the Crown, then moved into private practice as an attorney-at-law from 1997 to 2000.

Appointed Chief of Staff in the Office of the Prime Minister under Said Musa, a post he held until 2002 — the coordinating role at the top of the executive, held without an electoral mandate of his own.

On 5 March 2003 Fonseca won Freetown with 1,154 votes to the United Democratic Party's Doug Singh on 771, succeeding Jorge Espat. He was appointed Minister of Education, Youth, Sports, Culture and Labour, and became Chairman of the People's United Party.

In 2005 he took the Attorney General's portfolio, holding the state's principal legal office together with the education ministry until the government fell in 2008.

The PUP was heavily defeated. Fonseca held Freetown by fourteen votes — 1,369 to Michael Peyrefitte's 1,353 — one of only six PUP representatives in the country to keep a seat. In March that year he contested the party leadership and lost to Johnny Briceño, 310 votes to 330.

On 3 November 2011 Fonseca became Leader of the People's United Party and Leader of the Opposition, taking the leadership on his second attempt and only months before a general election.

The March 2012 general election returned the United Democratic Party to government by seventeen seats to fourteen. Fonseca held Freetown with 1,558 votes to Lee Mark Chang's 1,408.

Following the party's defeat at the November 2015 general election — at which he increased his own Freetown vote to 1,777 against Carla Barnett's 1,312 — Fonseca announced he would step down. A national convention was set for 31 January 2016 to choose his successor.

The PUP won the November 2020 general election. Fonseca took Freetown with 2,114 votes to Orson Elrington's 959 and was appointed Minister of Education, Culture, Science and Technology — the file he had left twelve years earlier.

In the 2022/23 academic year the Belize Education Upliftment Project — Together We Rise — opened at four government secondary schools on the southside of Belize City, covering 947 students, with tuition and fees removed entirely.

On 1 January 2024 Fonseca took Foreign Affairs and Foreign Trade in addition to education, after Eamon Courtenay left Cabinet at the end of 2023. On 2 February the National Assembly passed the amendment raising the compulsory school age to sixteen. On 28 September he delivered Belize's statement to the 79th United Nations General Assembly.

At the general election of 12 March 2025 Fonseca was returned for Freetown for the sixth time with 2,164 votes. In the Cabinet announced on 15 March he was given Foreign Affairs, Foreign Trade, Culture and Immigration, with Oscar Requena taking Education. In the reshuffle effective 17 November he resumed the education portfolio alongside foreign affairs.

The Belize Education Sector Plan 2.0 was launched to guide the sector from 2026 to 2030, against a reported education allocation of about $300 million for the fiscal year. Fonseca set out five core objectives, the first being equitable access and completion at every level of education.