Since the 1980s, Bainum has led businesses including four current or former publicly listed companies: Choice Hotels; HCR Manor Care, which operates nursing and assisted living facilities; Vitalink Pharmacy Services; and Sunburst Hospitality.

[1] He was the second of four children born to Stewart W. Bainum Sr., who was the founder of what became Choice Hotels and HRC Manor Care, and Jane Goyne.

[3] He received a Master in Business Administration from the UCLA Anderson School of Management in 1970, and taught at Southern College in the Tennessee River Valley.

[1][5] Bainum joined his father's Manor Care business in 1972 as the director of development and became senior vice president two years later.

[14] Of the 209 largest family influenced, publicly held companies from 1976 to 1996, the Bainums' Manor Care generated the second-highest average annual shareholder returns, second behind Warren Buffett's Berkshire Hathaway.

[18] George Washington University's School of Business and Public Management named Bainum CEO of the year in 1997, citing the company's annual revenue growth from $500 million to 2.4 billion under his leadership.

Bainum first ran for elected office in 1974, when he sought to unseat incumbent Maryland state Senator Victor Crawford.

[30] The Maryland Court of Appeals ruled that Bainum was ineligible to run, as he did not meet the state's residency requirements at the time.

[31] In 1978, he was elected to the Maryland House of Delegates, a post he held until 1982, serving on the Ways and Means Committee and as chairman of its Transportation Subcommittee.

In 1984, as a result of the lawsuit, a Montgomery County, Maryland, judge ruled that Burning Tree must give up its $186,000 yearly real estate tax exemption if it does not allow women.

[35] In 1986, legislation he sponsored that barred private clubs that exclude women from receiving state tax breaks became law.

[37] Bainum also sponsored a child support bill to require employers to deduct court-ordered payments, similar to how taxes are deducted,[38] as well as legislation requiring public officials to disclose withdrawals from failed savings and loans, increase penalties for illegal handgun use, make information public on toxic substances and hazardous waste, increase fees on heavy trucks to offset road repair costs,[32] and tax relief for low-income tenants.

[44] In 2018, Bainum and his wife committed to The Giving Pledge, a campaign to encourage wealthy people to contribute a majority of their wealth to philanthropic causes.

[48] After he was unsuccessful in this effort,[49] he committed $200 million of his own money to compete with Alden to acquire all of Tribune Publishing Company,[50] with the goal of finding "responsible buyers" for each of the Tribune-owned publications.

[51] He is the chairman of the Institute, which was named after former Baltimore County executive Ted Venetoulis, an advocate for local news who died in 2021.