This was the first time superstar Amitabh Bachchan sang a playback song for a movie. "Mere Paas Aao" was the first of many songs of a movie sung by him.
The film was originally titled after the famous con artist Natwarlal, but as he was offended by this his lawyer sent a legal notice to the producers and later the film was re-titled Mr. Natwarlal only days before its release.
Tito&Tony signed Amitabh/Rekha for "Natwarlal". When the shooting completed, they were told that the title is registered with another maker. T&T then opted for to add "Mr" to the title. But that maker lodged complaint with FPA that "Natwarlal" and "Mr Natwarlal" are same (as Natwarlal can be only a Mr).And he won again!! Traumatised T&T finally came up with the one -"Amitabh Bachchan In And As Mr Natwarlal",which didn't have any further objection.The censor certificate also bears the same.
Music composer Rajesh Roshan and dirrctor Rakesh Kumar encouredged Amitabh Bschchan to sing the song, Amitabh was relucant to singh and got nervous while recording. Rajesh Roshan was nomited for the music in the film.
After watching the crackling Amitabh-Rekha chemistry in Mr. Natwarlal Jaya Bachchan is supposed to have issued an ultimatum to Amitabh to stop seeing Rekha. No matter what the truth was about the rumoured affair, Amitabh did not sign any movie with Rekha after 1979. Ram Balram that released in 1980 had been in the making for four years. The only exception was Silsila which starred Amitabh, Jaya AND Rekha.