The company said on Sunday that Easterbrook "demonstrated poor judgment", since it prohibits senior management from having relationships with other workers, whether they were under their direct supervision or not, Efe news reported.
According to MarketWatch, Easterbrook sent an e-mail with a memo to employees acknowledging the relationship and said it was a mistake.
"Given the values of the company, I agree with the board that it is time for me to move on," he said in the letter.
McDonald's said it voted to approve Easterbrook's departure last week after conducting a thorough analysis of the situation, while the compensation's payment details will be known on Monday after the submission of documents to federal authorities.
After Easterbrook's dismissal, McDonald's board appointed Chris Kempczinski, who until now headed McDonald's USA and will take up his new position immediately as the new President and CEO.
Kempczinski was one of the key figures in the development of McDonald's strategic plan and has overseen the most comprehensive transformation of the company in the US in the history of the fast-food chain, the company's chairman, Enrique Hernandez, said in a statement.
"Steve brought me into McDonald's and he was a patient and helpful mentor," the new CEO said about Easterbrook.
The 52-year-old former CEO had led McDonald's since March 2015.
During his tenure, the company's shares nearly doubled in value but traffic to US restaurants has continued to stagnate.
Easterbrook also rolled back offerings including premium burgers and parts of an all-day breakfast menu after they slowed down restaurant operations.
Wait-times at McDonald's drive-thrus have climbed in recent years as the company's menu became increasingly complex.
Kempczinski, who helped implement many of the recent changes as head of US operations, said he would maintain Easterbrook's focus on technology as CEO and believes the company's investments will pay off.
McDonald's also faces more challenges at the US restaurants that drives its sales. Labour organizers and some lawmakers have called on the company to address workplace harassment issues and raise its minimum wage to $15 an hour.
( With inputs from IANS )