Speaking after seeing his side lose the 3rd ODI against Bangladesh, he said, "I've told the squad that it's time for me to have a break. I've been struggling with various injuries (shoulder, elbow and ankle), and my body's telling me it's time for rest. I've been a bit of a passenger in the field, and it's not fair on the rest of the team."
Johnston, who turns 34 next month, will play no part in Ireland's Friend Provident Trophy games, but hasn't ruled out featuring later in the year, in the Intercontinental Cup.
"It's been no secret that I've been contemplating this for some time. International cricket places a great strain not only on the body, but also on family life, and with a wife and two young children, I've got to prioritise. There are also the demands on my time with work. I've now got a terrific job with Architectural Hardware, who have been fantastic with their support."
"It's been a terrific time to have been involved with Irish cricket, and I'll always cherish the memories of the World Cup."
"I do feel I've still got a lot to offer, particularly in the 4 day format. I suppose I could still field in the slips, and I've been in good form with the bat, so could maybe concentrate on that aspect of my game more."
"I'll be sitting down with Phil Simmons this week and discussing my plans. I don't think I'm up to the strains of One Day cricket any more. These 3 ODI's in the space of 5 days has nearly killed me. It's only fair to give Phil a chance to build up a working relationship with William Porterfield now, rather than drop this on him in 6 months time."
Johnston will continue to play domestic cricket in Leinster, where he is captain of Railway Union. "I'm still going to be turning out for Railway, where the demands won't be as strenuous."
A Cricket Ireland spokesman said, "Trent has been an inspirational captain of the Irish team for 3 years. We fully understand the reasons behind his decision to take a break. We feel he still has a lot to offer on the playing side."
Johnston has captained Ireland to two successive Intercontinental Cup Final wins, against Kenya in 2005, and in 2007 versus Canada. He has a superb record in First Class fixtures, having taken 51 wickets at an average of 12.94, as well as scoring 368 runs at 36.80 in the11 games he has played.
Overall he has played 67 times for Ireland in all competitions, scoring 1332 runs at 21.48, and taking exactly 100 wickets at 24.43.
He is one of only two players to have taken a hat-trick for Ireland, doing so against Gloucestershire at Clontarf in May 2007.
