The fast-growing firm, which recently unveiled a 45 per cent surge in quarterly sales, said Dyson – who is also a non-executive at Betfair and Punch Taverns – would also serve as its senior independent director.
Dyson spent five years at M&S and had been tipped to take over the top job from former boss Sir Stuart Rose, but lost out to current chief Marc Bolland.
He left the high street stalwart in 2010 to become the chief executive of Punch Taverns and went on to run its Spirit managed pubs division following a demerger in 2011, before stepping down at the end of that year, although he retained a non-executive role at Punch.
Brian McBride, the Scots-born chairman of Asos, said: “Ian has a wealth of experience of being at the forefront of large UK-listed consumer-led businesses and has strong understanding of both the retail industry and of technology.
“He brings a new perspective to an already experienced non-executive team.”
Dyson’s appointment comes less than a month after another M&S veteran, Kate Bostock, abruptly quit the website.
Bostock, described as one of the most influential women in British fashion, spent eight years at M&S and admitted that Asos was “not the right place” for her.
Asos is targeting sales of £1 billion by 2015 and focuses on young shoppers aiming to emulate the designer looks of celebrities such as Cara Delevingne and Nicole Scherzinger.
M&S yesterday announced plans to close four unprofitable stores in Ireland, with the loss of 180 jobs, but insisted it was committed to the country, where it has been trading for 35 years.
Jonathan Glenister, head of M&S Ireland, said a flagship store is planned for Limerick in 2016, creating 250 jobs.
He added: “Despite taking this difficult business decision, we are very proud of our Irish business and will continue to invest in it for the future.”
However, the firm is also closing a warehouse operation in Northern Ireland, putting about 200 roles at risk.