After qualifying as an advocate, Hewagama assisted his cousin J. R. Jayewardene in his parliamentary campaign in the Kelaniya Electoral District by managing his election funds.

Donald Hewagama was the last to hold this unique position as subsequently after almost twenty years of his retirement, as no official filled this vacancy, the judicial services were separated for the three armed forces and three individual officers were appointed.

During 1971 JVP insurrection against then Sirimavo Bandaranaike Government, Lieutenant Colonel Donald D Hewagama was given an assignment to overlook the prevailing administration setup in the north central area of Sri Lanka and was sent with a convoy of armed military vehicles and on his way back from the destination he sighted a gathering of youth detained by the soldiers under emergency law in a road crossing near Warakapola area in the North Western province of the country.

[3] Following his retirement from the army at the age of 55 years, he served as the Colombo Harbour Magistrate[3] and functioned as the Judge at the trial of the then Member of Parliament for Hewaheta for the offence of smuggling gold bars to Sri Lanka.

When the offender was brought in front of the grand panel of military judges no one was there to defend this helpless man who assaulted the commanding chief of one of the world's mightiest armies.

He thought "This feeble sailor armed with an unloaded rifle stood for his land of birth and for all of its people and symbolically defended it from foreign invasion."