Lavery was elected to Dáil Éireann on his first attempt, at a by-election held on 17 June 1935 in the Dublin County constituency, after the death of Fine Gael TD Batt O'Connor.
Costello justified both decisions on the ground that Lavery was one of the Bar's top earners and had taken a considerable pay cut as Attorney General.
As Attorney General, he advised on several difficult issues, notably devaluation of the currency and fishing rights in Lough Foyle which were claimed by both Governments, North and South.
In 1961, on the retirement of Conor Maguire, Costello lobbied hard for Lavery to be appointed Chief Justice of Ireland, calling him with perhaps some exaggeration "the outstanding Irish legal figure of the last half-century".
He later lobbied, also unsuccessfully, for Lavery to be appointed a judge of the International Court of Justice (apparently the only time an Irish candidate was even considered).