The 76-year-old, fondly known as Q to friends and cricketers around the world, follows British journalist John Woodcock of The Times and former Australian captain-turned-writer and commentator Richie Benaud to the milestone. (Agencies)
Ahmed covered cricket's 1,000th Test, between Pakistan and New Zealand at Hyderabad, Pakistan, in 1984 and the 2,000th, between England and India at Lord's in 2011.
He has also clocked up 732 one-day internationals and eight World Cup tournaments.
"It's a great honour to complete 400 Test matches as a journalist and I am proud at the distinction," said Ahmed, a former left-arm spinner who has the distinction of dismissing all five of Pakistan's legendary Mohammad brothers in first class cricket in 1950s and 60s.
Ahmed was a promising player as a young man and almost made the Pakistan side for the country's first ever tour of the West Indies in 1958, but missed out and decided to fulfill his ambitions off the field instead.
"On not getting to play for Pakistan, I went to England and studied journalism but at that time it was tough to get a job, so I had to do odd jobs, but luckily I got a break in 1974 for British Broadcasting Corporation's Urdu service," he said.
Ahmed's Test "debut" came at the game's spiritual home Lord's, in the rain-dogged 1974 draw with England, a match made famous, or perhaps infamous, when the covers leaked, soaking the pitch and letting home spinner Derek Underwood run rampant to snare match figures of 13-71.
"From there I globetrotter to all cricket playing countries except for Bangladesh and it's a journey of great excitement and achievement," Ahmed said.
That 1974 series also saw a sublime double ton from the elegant Zaheer Abbas, who along with West Indian "Master Blaster" Viv Richards, Ahmed rates as his favourite batsman.
The game, and reporting on it, has changed enormously in Ahmed's time.
"There were no facilities like direct dialing, internet and emails -- we had to use telex, waiting for hours to send our stories," he said.
On the field the game has seen coloured clothing, umpire reviews with super slow-motion video and ball-tracking technology, and drop-in pitches -- a far cry from the pristine whites and matting pitches of the 1950s and 60s.
"The game has changed, in fact for the good," Ahmed said.
He says technology has helped get rid of the kind of bias -- or perception of it -- that led to the notorious on-field spat between England captain Mike Gatting and Pakistani umpire Shakoor Rana in 1987, a match Ahmed covered.
Cricket also gave him the opportunity of meeting legendary figures, notably the late South African leader Nelson Mandela.
"Meeting Mr Mandela was the highest point as his was an imposing personality and it was only because of cricket that I got that opportunity," said Ahmed.
He says he never suspected he would enjoy such a long career, which has seen him write for numerous outlets around the world, including AFP, Reuters and the Daily Telegraph, and has no plans to retire just yet.
"I didn't think I would do 25 Tests, so as long as I am fit I want to continue covering Tests," he said.
The 76-year-old, fondly known as Q to friends and cricketers around the world, follows British journalist John Woodcock of The Times and former Australian captain-turned-writer and commentator Richie Benaud to the milestone.