Lowth, who is also being made an executive board director, will join on 4 July.
He has held a range of roles in major firms, most recently chief financial officer and executive director at BG, which he joined in November 2013. He stepped down in February 2016, following its £35bn takeover by Royal Dutch Shell.
He held similar roles at AstraZeneca and ScottishPower.
Group chief executive Gavin Patterson said: "Simon brings a wealth of experience to BT, having served as CFO and a board director for a number of major international companies, and I'm thrilled he's joining.
"On top of his solid financial acumen, his strong organisational leadership and engineering experience at a number of leading infrastructure companies will be a real asset for BT.
"I'd also like to thank Tony Chanmugam for his tremendous hard work, commitment and the strong results that he has delivered during his career at BT. BT is a much stronger business today as a result of the phenomenal role Tony has played in BT's cost transformation and strategic direction. I wish him all the best in his future enterprises."