Special judge A S Shende convicted Panwar under Sections 302 (murder) and 326 B (Voluntarily throwing acid) of the IPC.
The court is likely to hear arguments on the quantum of sentence tomorrow.
Rathi, then 24, had died of multiple organ failure after Panwar threw acid on her on May 2, 2013 at the Bandra railway station here.
Panwar was Rathi's neighbour in Bhakra Beas Management Board Colony in Delhi.
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Mumbai police filed a 1332-page charge sheet against Panwar in April 2014 and also submitted a list of 98 witnesses after he was arrested from the national capital in January.
As per the charge sheet, Panwar, a hotel management graduate, threw concentrated sulfuric acid on Preeti at Bandra station here, as he was jealous of her career growth.
According to police, Rathi had secured a nursing job with the Ministry of Defence at the INHS Ashvini Hospital. Also, Panwar's parents often told him about his failure to get a job despite completing his education and would praise Preeti, who landed the job at the Navy hospital in Colaba.
Outside the court today, Panwar's mother Kailash demanded a CBI inquiry claiming her son had been falsely implicated.
"We have been implicated just because we are poor. I want a CBI inquiry into the case," she said.
Rathi's father Amar Singh Rathi hoped Panwar would get capital punishment.
"It took three years for us to get justice but I am happy that it has been finally delivered. I hope he gets death sentence," he said.
she got down from Garib Rath Express at the Bandra Terminus and took the same train back home.
The gruesome incident had set the local police on a wild goose chase as Panwar had covered his face at the time of the attack.
The Railway Police, which initially probed the case, had arrested another neighbour of Rathi, Pawankumar Gahalon, but set him free as there was no evidence against him.
Later, based on a Bombay High Court direction, the case was handed over to Mumbai Crime Branch, whose investigation led to Panwar and arrested him.
He said after Panwar's arrest, he had gone to the Crime Branch where the accused confessed about the whole incident.