Howard County is located in the U.S. state of Maryland. As of the 2020 census, the population is 334,529. Since there are no incorporated municipalities, there is no incorporated county seat either. Therefore, its county seat is the unincorporated community of Ellicott City. Howard County is part of the Baltimore metropolitan area.

The county is home to Columbia, a planned community between Baltimore and Washington, D.C. with a population of approximately 100,000, founded in 1967.

Howard County is frequently cited for its affluence, quality of life, and excellent schools. Its estimated 2020 median household income of $124,042 (~$147,702 in 2024) makes it one of the wealthiest counties in the US. Many of the most affluent communities in the area, such as Clarksville, Dayton, Glenelg, Glenwood, and West Friendship, are located along the Route 32 corridor in Howard County. The main population center of Columbia/Ellicott City is regularly ranked in Money magazine's Top 10 "Best Places to Live". According to data from the U.S. Census Bureau, Howard County ranks fourth in the nation for educational attainment, with an estimated 63.6% of residents 25 and over holding a bachelor's degree or higher. In 2022, the Howard County Public School System was ranked the best school district in Maryland.

In 2010, the center of population of Maryland was located in the Howard County town of Jessup.

The name of the county honors Colonel John Eager Howard, an officer in the "Maryland Line" of the Continental Army in the American Revolutionary War, commander notably at the Battle of Cowpens in South Carolina in 1781, among others. He was the fifth governor of Maryland, serving from 1788 to 1791.

Prior to the European colonization of what is now Howard County in the 1600s, the area served as farming and hunting grounds for Indigenous peoples including the Piscataway and Susquehannock peoples. The Maryland Historical Trust has documented Indigenous sites along the Patapsco, Patuxent, Middle and Little Patuxent River valleys. In 1652, the Susquehannock tribes signed a peace treaty with Maryland, giving up their provenance over the territory that is now Howard County. In 1800, the mean center of U.S. population as calculated by the US Census Bureau was found in what is now Howard County.

In 1838, Dr. William Watkins of Richland Manor proposed the "Howard District" of Anne Arundel County. After several adjournments, the area of western Anne Arundel County was designated the Howard District in 1839. The district had the same status as a county except that it was not separately represented in the Maryland General Assembly. In 1841, the county built its first courthouse in Ellicott City. At the January 1851 constitutional convention, Thomas Beale Dorsey submitted a petition led by James Sykes. A committee was formed with Dorsey, Bowie, Smith, Harbine and Ricaud. After several postponements, the district was erected officially as Howard County on July 4, 1851, after the approval of the new constitution at the election held June 4, 1851.

The plantations of modern Howard County used slave labor as early as 1690. At the time of the Underground Railroad, some Howard County residents assisted slaves who were escaping to freedom. This was particularly risky, as many prominent plantation families were Confederate sympathizers during the Civil War, contributing militiamen to the South to protect local interests. Maryland was exempt from the Emancipation Proclamation, later abolishing slavery in the update of the Maryland Constitution in November 1864.