Matías Alonso Vallejo

Born in Montevideo, Alonso started his career at local Club Atlético River Plate, signing with Peñarol in 2005 and not being successful at either club. He moved to Spain subsequently, first with RC Celta de Vigo's reserves in the country's Segunda División B. Alonso joined Real Murcia CF in 2007, being immediately loaned to another team in the third division, SE Ibiza-Eivissa. He appeared rarely for the main squad after returning, spending most of his spell with the B side. After Murcia's relegation from Segunda División, Alonso joined Granada CF in July 2010. In the following transfer window, without having made any official appearances for the Andalusians, he returned to his country, signing with C.A. Cerro on loan. On 1 September 2011, Alonso agreed to a one-year loan at Zamora CF in Spain's third tier. In February 2013, following a brief spell in China, he returned to his homeland and joined Juventud de Las Piedras. Alonso moved abroad again in September 2013, signing with A.S. Bari of the Serie B. Just three months later, however, he left Italy and moved to Defensor Sporting.

On 10 January 2025, Alonso was named as the new manager of the Uruguay under-17 team.

Alonso's older brother, Iván, was also a footballer and a forward. He too began his career at River Plate. His cousin, Diego Alonso, also played several years in Spain.