Lazzatti was born in
Bahía Blanca, but he grew up in the neighboring city of
Ingeniero White, where he played in local club Puerto Comercial as right inside forward. Santos Ursino was the central midfielder and captain of that team and then became his adviser. After a representative of
Boca Juniors (who was also Lazzatti's uncle) wrote a letter to the club recommending Ernesto, the club decided to pay the cost of travel to bring Lazzatti to
Buenos Aires by train. At 18 years old, the young promise arrived to the club and played three matches (a friendly one and two in the reserve team) before debuting in the senior squad in
1934, when Boca beat
Chacarita Juniors by 3–2 on April 8. In the year of his debut, Boca Juniors (managed by
Mario Fortunato) was champion, with 101 goals scored in 39 games. Lazzatti was the central midfielder alongside Enrique Vernieres and
Pedro Arico Suárez. In
1935 Boca won another championship. Lazzatti played only four matches for the
Argentina national team between 1936 and 1937. He was part of the squad that won the
1937 Copa América when Argentina beat
Brazil by 2–0 in the final at
Estadio Gasómetro. Lazzatti was part of the midfield line along with
Antonio Sastre and
Celestino Martínez. In Boca Juniors, Lazzatti formed a midfield line with
Carlos Sosa and
Natalio Pescia in the squad that won the
1943 and
1944 championships.
River Plate's ''
La Máquina was active during this period, and the Boca team is sometimes regarded as one of the club's best teams ever.'' Lazzatti played a total of 14 seasons in Boca Juniors, totalizing 503 matches and 7 goals scored. He was never sent off in his whole career. When he became a free agent at the age of 32, Lazzatti refused to play for other team in Argentina, so Uruguayan player and former teammate
Severino Varela offered him to play at
Danubio. Lazzatti accepted and joined the club, playing there until his retirement in 1948 at 33 years old. His performances were praised by Uruguayan media, remarking that "his quality is as expected". Despite that success, he never returned to management, pursuing a career in sport journalism, working in
El Gráfico when the magazine was directed by
Dante Panzeri. When Panzeri left in 1962, Lazzatti followed him. Nevertheless, Lazzatti continued his career as journalist writing for newspaper
La Prensa and in the TV show
Deporte con Opinión in
Canal 7. ==Titles==