During the Polish Defensive War of 1939, Kalenkiewicz served in the staff of the Suwałki Cavalry Brigade, as part of the Narew Independent Operational Group.

He took part in his units delaying actions in northern Poland, but in mid-September, he volunteered for the improvised Polish 110th Uhlan Regiment under ppłk.

The unit got surrounded by the Red Army in the Biebrza River area and suffered serious casualties, but managed to break through the enemy defences.

In November 1939 Kalenkiewicz reached Warsaw with a mission from Dobrzański to Gen. Michał Karaszewicz-Tokarzewski, the commander of the Służba Zwycięstwu Polski, the first major resistance organization formed in all parts of Nazi and Soviet-occupied Poland.

He also repeatedly proposed Gen. Kazimierz Sosnkowski to organize a group of paratroopers to be transported to occupied Poland for easier contact with the underground.

In early May Sosnkowski admitted Kalenkiewicz to his own staff and on June 25 both were evacuated to Great Britain due to the French capitulation.

Considered a member of the informal faction of young officers insisting on reforming the Polish Army in exile into a highly trained specialized force rather than cannon fodder, Kalenkiewicz co-authored several memorials urging the Polish authorities to reform the standard infantry units into airborne units capable of both armed reconnaissance and diversion, and possible help in the envisioned all-national uprising in the German-occupied country.

By accident the pilot landed in the areas annexed by the Third Reich rather than in General Government (GG) and all the members of the team were arrested immediately upon their arrival.

There Kalenkiewicz met with Gen. Stefan Rowecki (Grot), who accepted him into the Department of Operations of the staff of the Association of Armed Struggle, the predecessor of the Home Army.

As a rule of thumb, all Cichociemni were promoted one grade upwards on their arrival to Poland and so Kalenkiewicz rose to the rank of Major.

In the film Hubal (1973) directed by Bohdan Poręba the role of Captain Maciej Kalenkiewicz was played by the actor Tadeusz Janczar.