Dharmasena started his cricketing career as a teenager at Nalanda College Colombo. His first foray into international cricket was in 1994 against South Africa.

His obscure action made him perfect for bowling in one-day matches, yet Dharmasena also proved a useful batsman, especially after he was investigated in 1998 by the ICC for overstretching his bowling action to illegal proportions. Having been cleared in July 2000, he played for the one-day team on several occasions since, but rarely played Test cricket.

Dharmasena was the 59th Sri Lankan cricketer to receive a Test cap (Sri Lanka v South Africa at Colombo Sinhalese Sports Club 1993).

Kumar Dharmasena has the record for playing the most number of ODI innings before being dismissed for a duck (72 innings).

He along with Dulip Liyanage set the record for the highest 8th wicket runstand for Sri Lanka in ODI cricket (91).

Following his retirement from playing in November 2006, Dharmasena announced plans to become a competitive umpire, as he wished to remain "close to the game which I love so dearly". Prior to his retirement, he had already umpired several domestic matches in the Sara Trophy, the major first-class cricket tournament in Sri Lanka. He umpired his first international match in 2009, overseeing the one-day international between India and Sri Lanka at the Rangiri International Stadium in Dambulla: he remains the youngest ever Sri Lankan to umpire any international match. He umpired at the 2011 Cricket World Cup, and was appointed to the Elite Panel of ICC Umpires later that year.^[1] Dharmasena was named the Umpire of the Year at the 2012 ICC Awards, for which he received the David Shepherd Trophy.^[9] As of 2017^[update], he is the only Sri Lankan to receive this precious award.

He was selected as one of the twenty umpires to stand in matches during the 2015 Cricket World Cup^[10] and umpired in the final.^[11] In doing so he became the first to play in and to umpire World Cup finals.^[12] He stood in the final of the 2016 ICC World Twenty20.^[13]

Kumar Dharmasena has worked as an umpire in 18 matches of ICC World T20, the most by any umpire from Sri Lanka.^[14]

DRS which is the shortened form of Decision review system, is often humorically called as Dharmasena Review System for his umpiring. Even though he was ranked number 1 in the world for his umpiring, has slightly struggled in providing the judgements in recent past. Especially in the English cricket team in Bangladesh