Early Life

Theekshana was raised by his grandmother and hailed from Siddhartha Vidyalaya, Sedawatta. Two of the old boys from St. Benedict's College, Colombo requested Theekshana's guardian to shift their base to Kotahena so that Theekshana could play school cricket and represent St. Benedict's College. The St. Benedict's Cricket Wing volunteered to manage his finances for the first two years and later St. Benedict's College offered him a scholarship. He repaid his faith by winning the All Island Best All Rounder Award in his final year at St. Benedict's College.

Military Career

Theekshana joined the Sri Lanka Army, served as a private in the Gajaba Regiment after basic training and played for the Sri Lanka Army cricket team.

He was promoted to sergeant after the 2022 Asian Cup Championship.

Domestic and T20 Franchise Career

After impressing with the ball at school, he was called up by the National Cricket Academy for trials at the R. Premadasa Stadium. He then impressed National Cricket Academy spin bowling coach and director of cricket Simon Willis with his control, accuracy, bounce and variations, as well as his Ajantha Mendis-like bowling action. However, his body weight came under scrutiny as he weighed 105 kilograms and 140 folds of skin at one point and also had difficulty passing the fitness tests as he took 10.1 minutes to complete the two-kilometer tests and also performed below average in the YOYO test conducted by Sri Lanka Cricket. He was also dropped from the Sri Lanka U-19 squad for the 2018 U19 Cricket World Cup due to fitness issues. According to some sources, Theekshana weighed around 117 kilograms during his U19 days.

Theekshana made his List A debut for Colts Cricket Club in the 2017–18 Premier Limited Overs tournament on 14 March 2018. He made his first-class debut for Colts Cricket Club in the 2018–19 Premier League tournament on 7 December 2018. In October 2020, he was drafted by the Jaffna Stallions for the inaugural edition of the Lanka Premier League. He joined the Sri Lanka Army in 2020 and saw a complete transformation as he started following strict diets and worked hard to improve his fitness. He subsequently also reduced his weight in line with the expectations placed on him to pursue his cricketing ambitions. By 2020, he had lost 22 kilograms and halved the number of skin wrinkles. He also improved his two-kilometer run within 10 minutes and improved his YOYO test maximum from 16.1 to 19.2. He trained under Ajantha Mendis, who was his first coach in the Sri Lanka Army when he joined the Army's U-23 team. Due to his impressive performances in the 2020 Army Commander's Cup, he was selected for the 2020 Lanka Premier League. He initially did not make it to the 2020 Lanka Premier League draft and was categorized in the additional category, making him ineligible to play. However, on the recommendation of Thisara Perera, he was promoted from the supplementary category to the emerging category in the Jaffna Stallions squad for the 2020 LPL season after the application was accepted by the SLC.

In August 2021, Theekshana was named in the SLC Blues team for the 2021 SLC Invitational T20 League tournament. In November 2021, he was selected to play for the Jaffna Kings following the 2021 Lanka Premier League player draft. He finished the tournament as the top wicket-taker of the 2021 Lanka Premier League.

In February 2022, he was purchased by the Chennai Super Kings as part of the auction for the 2022 Indian Premier League tournament. During the 2022 Tata IPL edition, he broke Rohit Sharma's record and became the youngest bowler ever to take a four-wicket haul in an IPL match when he scored against Royal Challengers Bangalore at the age of 21 years and 255 days reached the milestone.

In July 2022, he was signed by the Jaffna Kings for the third edition of the Lanka Premier League. In July 2022, he was signed by the Trinbago Knight Riders for the 2022 Caribbean Premier League.

In August 2022, he was selected by the Johannesburg Super Kings for the inaugural edition of the unnamed CSA T20 League in South Africa, scheduled to take place in 2023.

International Career

Following Theekshana's strong performances in the SLC Invitational T20 league, he was named in the Sri Lanka One Day International (ODI) and Twenty20 International (T20I) squads for the series against South Africa. He made his ODI debut for Sri Lanka against South Africa on September 7, 2021. In the match, he took a wicket with his first ball in ODI cricket. He took 4 wickets for 37 runs and helped Sri Lanka win the series 2-1. He was the first Sri Lankan international cricketer to be born in the 21st century. He made his T20I debut on September 10, 2021, also for Sri Lanka against South Africa. Later the same month, Theekshana was named in Sri Lanka's squad for the 2021 ICC Men's T20 World Cup. He finished the tournament as the second highest wicket-taker for Sri Lanka with eight scalps.

In June 2022, he reached a career-best eighth place in the ICC Men's T20I Player Rankings for bowlers.

In July 2022, he was included in Sri Lanka's Test squad for the second match against Australia. He made his Test debut for Sri Lanka against Australia on July 8, 2022.

In August 2022, he was included in Sri Lanka's squad for the 2022 Asia Cup. Along with Wanindu Hasaranga, Theekshana made it to the spin bowling department where Sri Lanka eventually won the tournament for the sixth time. Theekshana made a consistent wicket-taking option for skipper Dasun Shanaka where he finished the tournament with 6 wickets for 162 runs and an economy of 6.75.

Controversy

Chennai Super Kings fans expressed their sadness and vented their frustration in disbelief by posting hashtags #Boycott_ChennaiSuperKings and #DontNormalizeTamilGenocide on various social media platforms soon after Chennai Super Kings bought Theekshana during the IPL 2022 auction , as he had served in the Sri Lanka Army.