Slaoui will step down from the board at the end of March 2017, following the companies full year results. He will, however, remain an adviser until the end of June next year.
Slaoui joined GSK in 1988 and became a board member in 2006.
Shares in the company were down 1.1 per cent following the announcement today and have struggled in recent years.
Earlier this year GSK chief executive Sir Andrew Witty announced he would stand down from the helm of the pharma giant after nearly a decade.
Witty will retire on 31 March 2017, and it's expected his replacement will be announced before the end of the year.
Glaxo chairman Philip Hampton said:
On behalf of the board, I would like to thank Moncef for his great contribution to the company. He has been personally involved in the discovery and development of some of our most innovative vaccines and spearheaded the overhaul of GSK's pharmaceutical R&D.
In April, GlaxoSmithKline reported sales rose to £6.23bn, up 11 per cent, in the first quarter on last year, generating core earnings per share (EPS) of 19.8 pence.
Read more: Break-up a bitter pill for Witty
Witty is keen for the company to split its focus between consumer health and new drugs, though some activist investors have claimed it's an inefficient way to run the drug giant and it should be broken up.