In March 1969 he moved to Buenos Aires and entered the group “Los Bambis”, and together recorded for the first time for Columbia Broadcasting System (CBS). The vinyl recording was named Los Bambis también cantan. CBS producers soon saw Hoffmann’s potential as a solo singer; in November 1969 he recorded the songs Fui un soñador (I was a dreamer) and Te llamo para despedirme (I call you to say goodbye), produced and written by Francis Smith.

This single began to have radio airplay in December, and on 1 January 1970 Denis first performed on TV, in the programme “Casino Philips”, hosted by Andrés Percivale and broadcast by Canal 13 de Buenos Aires. During that same month, Canal 13 hired him for another programme, Sábados Circulares hosted by Pipo Mancera. In March, Denis travelled to México D.F. to participate in the contest “Primer Festival de la Canción Latina” (First Festival of the Latin Song), event in which he reached the 5th place with the song Yo te amo como entonces (I love you like before); curiously this song was never released. Once he returned to Argentina, he was enlisted to do the Servicio Militar Obligatorio, and assigned to the Batallón de Comunicaciones 181 in Bahía Blanca. So, he alternated his military duties with his public performances and TV performances.

In 1972, due to the success of a song written by him, he co-starred in the film Me enamoré sin darme cuenta (I fell in love without noticing it) with Alicia Bruzzo and Luis Brandoni, directed by Fernando Siro. On 31 July 1974 he sang in the Teatro Opera, being the first pop/ballad singer to perform in that prestigious theatre. His performance was backed up by a 36 musicians-orchestra directed by Jorge Calandrelli.

Some of his most popular ballads and romantic songs are: Yo nunca supe más de ti (I never knew anything else about you) from the homonymous album in 1973, Cada vez que sale el sol (Every time the sun rises) from the album of the same name (1977), Cómo estás, querida (How are you, darling) from the album Afectos (1985), Te quiero tanto (I love you so much) included in the album Imágenes (1986), and Un poco loco (A little crazy) from the homonymous album in 1991. He also recorded cover versions in Spanish of songs originally recorded in English, such as: California somnolienta (California dreamin’, by The Mamas and the Papas) from the album Al Estilo de… Sergio Denis II (1981), Pipas de la paz (Pipes of peace, by Paul McCartney) included in the album La Humanidad (1984), Nada va a cambiar mi amor por ti (Nothing’s Gonna Change My Love for You, by Glenn Medeiros) and Así fue nuestro amor (Annie’s Song, by John Denver), both included in his album Afectos. He won many awards, being the best-selling Argentine singer-songwriter between 1985 and 1994.

In 1975 Denis broke free from CBS Columbia, and in mid-1976 he recorded a sole album for a small recording company named TK. The following year he was signed by PolyGram (now Universal); with this company Denis recorded most of his greatest hits until July 1990. In 1991 he signed a contract with EMI Odeón, where he recorded four more albums until 1995.

In 2007, Sergio Denis was dating a girl named Melisa Durán, when she was selected to participate in the TV programme Gran Hermano 2007 broadcast by Telefe. Just a week after entering the house, Durán cheated on Denis with another contestant of the reality show. Her infidelity caused both the breaking up of the couple and her expulsion from the house, with more than 80% of votes against her.

On 14 May 2007, while on tour in Asunción, Paraguay, he suffered a decompensation that caused a cardiopulmonary arrest. Denis was admitted to the Sanatorio San Roque of that city unconscious and without vital signs, but he was resuscitated. The following day, the doctors disconnected him from the artificial respirator, and after a period of observation, he returned to his normal life.