The Agro-chemicals giant boss said the firm seriously weighed Monsanto's advance, but ultimately the underlying offer was "completely inadequate."
"On the one hand we took a look at this proposal, because we must - it is a public company. But on the other hand, it is woefully short across all the important dimensions for us to give it further serious consideration," Mack told CNBC.
Syngenta rejected a $45 billion takeover offer from Monsanto in May, saying the offer undervalued the Swiss firm and did not fully take into account regulatory risks. The company's board had unanimously rejected a 45-percent cash offer by Monsanto that would value it at 449 Swiss francs ($486.35) per share.
Responding to Syngenta's earnings announcement, Monsanto CEO and chairman, Hugh Grant said the results confirm the group lacks a long-term vision and plan that would match the "very attractive" proposal Monsanto made earlier in the year.
"We've made a serious proposal at full and fair value that would provide their shareholders with a 43 percent premium and significant further upside potential through ownership in the combined company," Grant said in a renewed bid to sway Syngenta shareholders.
"It's clear that there is broad support for our strategic rationale. This is accompanied by Syngenta shareholders' frustration with its refusal to at least engage in dialogue with Monsanto," Grant added.
Read More Syngenta rejects second takeover proposal from Monsanto
Syngenta's Mack said he planned to speak to activist investors pushing for a reconsideration of the deal in the coming weeks, to give them confidence of the standalone business prospects of the group.
"We took a serious look at this thing and the second half of April and we gave a firm and clear answer that there is no negotiation table to sit down to because this unsolicited proposal failed frankly on every dimension that it would need to succeed in order for us to go any further. I don't think negotiate is the step right now because there is nothing to negotiate," he added.
"Syngenta is an immediate answer to their really long term problems. I don't think it's incumbent on us to solve Monsanto strategic problems," Mack told CNBC.
Syngenta shares closed around 0.8 percent down on Thursday, at the bottom day of the SIX, in a strong day for Swiss earnings that saw the index close around 1.0 percent higher.
The company recorded sales of $7.63 billion, were down 10 percent from a year earlier but up 3 percent when adjusted for currency swings, also exceeded the market view.