Following his graduation and marriage in 1919, he relocated to Beirut to join his family, who had already settled there. At that time, Beirut had become the capital of the newly established
Greater Lebanon under the
French Mandate (1920–1943), and Altounian was appointed as an architect within the Ministry of Public Works. The French authorities undertook an ambitious program of urban modernization in Beirut's city center during the 1920s and 1930s, transforming the area around what would become
Place de l'Étoile (Nejmeh Square) into a
Haussmann-inspired star-shaped plaza, a plan drafted by
Camille Duraffourd and executed between 1926 and 1933. Altounian became one of the central architects commissioned to design buildings around the square, Altounian's most celebrated work is the
Lebanese Parliament Building, situated on
Nejmeh Square in central Beirut. Commissioned during the French Mandate in 1933 and completed in 1934, the building serves as the seat of the
Parliament of Lebanon. Advised by the authorities to design the building in the spirit of Lebanese tradition, Altounian undertook research visits to the Emirs' palaces in the
Chouf Mountains, drawing on local vernacular forms while synthesizing them with the Beaux-Arts vocabulary of his Parisian training. He also drew on Oriental styles then being interpreted by architects working in Paris, Istanbul, and Cairo. The building integrates Beaux-Arts compositional principles with distinctly Levantine decorative elements, including twin and triple arch windows, and a limestone façade adorned with recessed panels, arched openings, and tiered muqarnas (stalactite vaulting). The interior features a reinforced concrete frame supporting a cupola of twenty meters in diameter covering the chamber of deputies. Before the
Lebanese Civil War, the building also housed the National Library; the library's collection of approximately 20,000 volumes, including ancient manuscripts, was partly destroyed during the conflict. Also located on
Nejmeh Square, the Al-Abed Clock Tower was designed by Altounian and completed in 1934. The tower was a gift to the city of Beirut from Michel Abed, a Lebanese-Brazilian émigré businessman, and its four-faced clock became an iconic element of the square's streetscape. Altounian's portfolio extended across a diverse range of civic, religious, and residential commissions. In 1937, he designed the Armenian Sanatorium of Azounieh in the Chouf region, a healthcare facility serving the Armenian community in Lebanon. That same period saw the completion of the Abu Bakr Al-Siddiq Mosque at the Port of Beirut and the Al-Daaouk Palace in Hamra. In 1942, he designed Asmahan Palace in Aley, and in 1947 the Church of the Paulist Fathers in Harissa. Other works he completed for the Armenian community in Lebanon included the Cathedral of Saint Gregory the Illuminator in Antelias (1939–1940). In 1946, he designed the
Evacuation Stele (Lāwḥat al-Jalāʾ) at Nahr el-Kalb, north of Beirut, commemorating the withdrawal of all foreign troops from Lebanese territory on 31 December 1946, during the presidency of
Bechara El-Khoury. == Work abroad ==