Daniel Leasure was born and raised in Westmoreland county, Pennsylvania. He was educated at the Greensburg Academy and graduated from Jefferson Medical College in 1846. In 1849 he and his wife, Isabel, settled in New Castle, Pennsylvania where he successfully practiced medicine until the outbreak of the War of Rebellion.

Upon learning of the capture of Fort Sumter, Dr. Leasure organized two military companies in the vicinity of New Castle. Shortly afterwards, he was given a colonelcy in the IX Corps which he held throughout most of the war. He was the first commander of the 100th Pennsylvania Regiment, known as the "Roundheads" because it was recruited from descendants of the followers of Oliver Cromwell.

Colonel Leasure and his unit first saw action at the Battle of Secessionville, South Carolina on June 16th, 1862. Following that battle, he was at Second Bull Run (2nd Manassas) and the Battle of Chantillty in Virginia. He was wounded at Bull Run and subsequently missed the Maryland Campaign, but returned in time for the Battle of Fredericksburg where he took command of the 3rd Brigade, better known as Leasure's Brigade of the 1st Division of the IX (9th) Corps.

Leasure continued to see action at the Battles of the Wilderness, Spotsylvania and the sieges of Vicksburg, Knoxville and Blue Springs. At Spotsylvania, division commander Brig. Gen. Thomas G. Stevenson was killed and Col. Leasure took command until relieved by Maj. Gen. Thomas L. Crittenden 3 days later. A few days after being relieved, Col. Leasure was again wounded and never returned to field command.

He was mustered out of the service on August 30, 1864 and returned to Pennsylvania where he practiced medicine in the borough of Darlington. In 1865, he was breveted to the rank of Brigadier General, became a trustee of Greensburg Academy and, later, served in the State Legislature.

In 1878, Leasure moved to the city of St. Paul, Minnesota where he died 8 years later. His remains were returned to Pennsylvania and he was interred in New Castle.