Grigonis is a basketball player, born on April 26, 1994 in Kaunas, Lithuania. He is a guard-forward who last played for CSKA Moscow.
Before starting his professional career, Grigonis played in the NKL with the Zalgiris-Arvydas Sabonis school for four seasons.
On 15 May 2013, Grigonis was brought into the main Zalgiris roster for a game against BC Nizhny Novgorod in the VTB United League.
For the 2013–14 season, Grigonis he was loaned to the Spanish second division team Penas Huesca.
On 14 August 2014, Grigonis signed a two-year deal with Bàsquet Manresa of the Liga ACB. After spending two seasons with Manresa, he signed a "2+1" deal with Iberostar Tenerife on 28 July 2016. He was named the Final Four MVP of the Basketball Champions League 2016–17 season. On 13 July 2017, he signed with German club Alba Berlin.
In July 2018, he returned to Zalgiris Kaunas when he signed a three-year contract. He averaged 11.5 points, 2.6 rebounds and 2.0 assists per game in the first 10 games of the 2019–20 season.
On 12 June 2021, Grigonis signed with CSKA Moscow. He averaged 8.5 points, 1.9 assists, and 1.4 rebounds per game in Euroleague.
Grigonis represented Lithuania in the U–16, U–18, U–19 and U–20 youth tournaments. He led his team to two silver medals and a bronze medal while participating in four tournaments. In 2014, coach Jonas Kazlauskas included Grigonis in the preliminary 24–player candidate list for the senior national basketball team. Though, he was invited to the national team training camp for the first time only in 2016 and immediately qualified into the Olympic roster.