After passing the Philippine Bar Examination in 1915, he worked as a lawyer in his home province.

[1] In 1931, he was appointed as a judge,[4] and spent the rest of the decade serving in the Visayas, Laguna and eventually in Manila.

[5] During the Japanese occupation, he was appointed as an associate justice of the Court of Appeals by President Jose P.

[1] After his term in the Senate, Sebastián was appointed by President Manuel Roxas to become Envoy-Extraordinary and Minister-Plenipotentiary to China in 1948.

He was later appointed by President Elpidio Quirino as ambassador to Italy and the Holy See, France, Belgium, the Netherlands, the United Nations Office at Geneva and ultimately Indonesia before retiring from the diplomatic service in 1954.