In July 1924, returning to the Submarine Base at New London, he served as an instructor in Diesel Engineering for one year, assigned to USS Chewink (AM-39).

In November 1925, he was assigned additional duty as technical adviser to the Peruvian Naval Commission at the Electric Boat Company, Groton, Connecticut, and when detached in October 1926, he commanded the submarine USS S-46 (SS-157), based on Coco Solo, Canal Zone, until May 1929.

Additionally, he was assigned as Liaison Officer / Engineer when the submarine USS O-12 (SS-73) was converted for use in the under-ice attempt to circumnavigate the Arctic Icecap and voyage to the North Pole.

Modifications to the submarine were extensive, and contained innovations of a telescoping conning tower, an ice drill, an incorporated diving bell and an Air Lock, designed by Simon Lake.

The submarine was designated Nautilus and leased for one dollar to Lake and Danenhower, Inc., of Bridgeport, Connecticut, for use on Hubert Wilkins's and Lincoln Ellsworth's Arctic Expedition.

Transferred to duty as Damage Control Officer on the staff of Commander Cruisers, Scouting Force, USS Chicago (CA-29), he served in that assignment until June 1938.

He received a letter of commendation from the President of the United States, Franklin Delano Roosevelt for the success of this extremely difficult operation.

He was assigned for a tour of shore duty in the Fleet Maintenance Division, Office of the Chief of Naval Operations, Navy Department, and served until July 1944.

The Tenth Fleet was dissolved in June 1945, and Rear Admiral McCann was transferred to duty on the staff of the Commander in Chief, Atlantic Fleet, and was subsequently designated Commander, Task Force 68, in the light cruiser USS Philadelphia (CL-41) during the Presidential trip to Berlin for the Potsdam Conference, July 4, to August 8, 1945.

He personally reported to President Harry S. Truman the news of the successful atomic bombing of Hiroshima, Japan.