He was in command of the steam sloop of war USS Kearsarge during her historic 1864 action off Cherbourg, France, with the Confederate sea raider CSS Alabama.

For his gallantry in action he was allowed to have command of the schooner USS Union, which had been captured at Tampico in November 1846 and was taken into service, but she was poorly equipped and was lost on a reef off Veracruz on 16 December 1846.

He was executive of the sloop Saratoga in the Gulf of Mexico in 1848–49, at the Boston Navy Yard in 1849–50, and in the frigate St. Lawrence of the Pacific Squadron in 1851–55.

While attempting to refloat the ship, Winslow was badly injured by a flying chain link and forced to return home late in the year to recover.

Detached from the Mississippi Squadron, Winslow returned to his home in Roxbury, Massachusetts, early in November and was confined to bed there for a month attempting to regain his health.

On 5 December 1862, orders arrived directing him to proceed via New York City to the Azores, where he was to assume command of the screw sloop USS Kearsarge.

However, when he reached that island on Christmas Eve, he found that Kearsarge had sailed to Spain for repairs, and he was forced to remain at Fayal until spring.

In Kearsarge, he cruised among the Azores seeking Confederate commerce raider CSS Alabama until autumn when he shifted to European waters.

[3] At Ferrol, Spain, Winslow learned that CSS Florida was at Brest, France, undergoing overhaul; he promptly sailed for that port to prevent her from slipping out to sea again.

While keeping track of the progress of the repair work on the Southern warship through the U.S. diplomatic and espionage network, he also made runs along the coast of western Europe, checking on rumors of other Confederate raiders in the area.

In January 1864, Kearsarge returned to Cádiz for naval stores and repairs, and while she was away from Brest, Florida put to sea on 18 February.

On 12 June, while moored in the Scheldt off Vlissingen (Flushing), Winslow received a telegram informing him that Alabama was at Cherbourg, a French naval port.

On Sunday, 19 June 1864, he was lying 3 miles (5 km) off the eastern entrance of the harbor when the Alabama came out, escorted by a French iron-clad and the English yacht Deerhound.

The sides of the Alabama were torn out by the shells, and at noon, after the action had continued for one hour, she headed for the shore to get into neutral waters, then 5 miles (8 km) distant.