BT had said in December it planned to make changes to the way it decides management pay after some of its shareholders questioned Patterson's bonus for 2018-19.
Investors had also complained after BT gave a more than 1 million pound bonus to Patterson in 2017-18, during which time the company's stock tumbled 30%.
"The (pay) committee and Gavin agreed that a reduction of the total bonus outcome by 50% would be the right thing to do and in the best interests of all stakeholders," BT said.
Patterson's bonus for 2018-19 will now be 572,000 pounds compared with 1.3 million pounds a year earlier. His total salary for the year was 1.72 million pounds, BT's annual report showed.
Patterson left BT at the end of January, handing over to Philip Jansen, a former Worldpay chief executive.
The telecoms giant had said a new leader was needed to restructure the company.
Patterson, who ran BT for almost five years, announced 13,000 job cuts last year in an attempt to get to grips with a host of problems including intense competition, an underperforming IT services unit, a huge pension deficit and criticism of its broadband plans.
But a failure to hit a revenue target and a forecast for no growth in profit for the next couple of years sent its shares to six-year lows.
(Graphic - BT Group Underperforms European Rivals,
(Reporting by Noor Zainab Hussain in Bengaluru; Editing by Mark Potter)