Mills, 28, and Clark, 36, hold a 20-point lead going into the finale, after finishing second and third in Tuesday's races.
The duo missed out on gold at London 2012, taking silver as New Zealand edged them out in the medal race.
"It feel like we've been sailing forever to get this gold," said Clark.
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"It's completely surreal. We've sailed out of our skin in a marathon event."
The pair have dominated throughout the event, winning three races and finishing in the top eight in all 10.
However, a medal is now out of reach for male 470 pair Luke Patience and Chris Grube, who are sixth going in to the medal race.
Alain Sign and Dylan Fletcher stand a chance of claiming a medal in Thursday's 49er medal race, sitting fourth, but Charlotte Dobson and Sophie Ainsworth are seventh.
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The Uganda Wildlife Association (UWA) has said the 832 pieces of ivory could be worth up to $6.7m but other estimates put the value lower.
Experts say some of the ivory is believed to have come from elephants poached outside Uganda.
There is a huge demand for African ivory in Asia for use in ornaments.
The ivory was apparently heading for the Kenyan port of Mombasa.
Some of the tusks had been broken up and covered with plastic bottles in crates labelled as material to be recycled.
The UWA told the BBC's Catherine Byaruhanga that Uganda is increasingly being used as a transit country by poachers who kill elephants in states such as South Sudan or the Democratic Republic of Congo.
Uganda is believed to have about 5,000 elephants and this haul would represent the tusks from about 400 animals.
Last week, some four tonnes of ivory was found in Mombasa after coming from Uganda.
According to the UWA, the rate of elephant poaching is relatively low in Uganda, with an estimated 20 elephants killed for their ivory across the country in 2010.
After a sharp fall in the number of elephants in Africa, the Convention on International Trade in Endangered Species (Cites) banned the trade in ivory in 1989
But in recent years poaching has increased across sub-Saharan Africa with criminal gangs slaughtering elephants for ivory markets in Asia.

MP Damian Collins has asked the body to investigate whether the payment breached bribery laws.
The £3.9m payment was made to the FIA for entering into an agreement with the teams and sport's commercial arm.
The FIA says the payment was remuneration "for its regulatory role" and denies wrongdoing.
Collins, chairman of the Culture, Media and Sport Parliamentary select committee, said he was "very concerned" about why the sport would need to make a payment to its governing body and regulator as part of the so-called Concorde Agreement, which was signed in 2013.
"That's why I've written to the Serious Fraud Office (SFO) asking them 'do they feel there was a breach of the Bribery Act and does it warrant investigation'?" he told ITV.
An SFO spokesperson told BBC Sport: "The Serious Fraud Office is reviewing material in its possession in relation to these allegations. All matters referred to the SFO are assessed against criteria to establish whether they may fall within its remit to investigate."
The FIA confirmed in a statement that it had received the payment and explained: "The Concorde Implementation Agreement entered into by the commercial rights holder of Formula 1 and the FIA in 2013 introduced a new governance structure for Formula 1 and redefined certain conditions applicable to their relationship, in particular to ensure that the FIA be properly remunerated for its regulatory role.
"Within this agreement, a lump sum payment of $5m (£3.9m) was made to the FIA as part of the global consideration received in connection with the renegotiation of the terms of the agreements between the commercial rights holder and the FIA, and of the Concorde Agreement, at that time.
"Following its approval, the Concorde Implementation Agreement came into force and this sum was paid to the FIA and properly accounted for.  No individual received any payment out of this sum. Any allegation to the contrary would be defamatory."
Great Britain's Hannah Mills and Saskia Clark will win Olympic gold in the women's 470 sailing if they finish Wednesday's medal race.
[NEXT_CONCEPT]
Ugandan officials have seized some two tonnes (4,600lb) of ivory - one of the country's biggest such hauls for many years.
[NEXT_CONCEPT]
The Serious Fraud Office is "reviewing material" relating to a payment made by Formula 1's commercial rights holder to the sport's governing body the FIA.