Generations of Dudley families have been introduced to hundreds of species of animals since the zoo opened its doors in 1937.
Former favourites at the zoo included elephants and killer whales but the present iterations of the animal houses and enclosures are ensuring rare species survive.
Dudley Zoo director Derek Grove joined more than 40 years ago as an animal keeper and now is helping shape the future of the tourist attraction.
He told the Express & Star: "It is so wonderful to see the crowds back here after the last few years. We are celebrating our 85th birthday and are delighted people have turned up in their 1930s clothes like the zoo's first ever visitors would have worn.
"The idea of the zoo was to help safeguard the future of the castle and that has been the case. I've been here for more than half of the zoo's life and I've seen a lot of changes.
"Before the emphasis was on iconic animals, like elephants and even killer whales in the 1970s, but things have changed, we should never have had killer whales, but now we concentrate on species which have a high conservation value.
"We were dealing with the consequences of global warming and deforestation years ago but now they are very much on the agenda."
An exhibition charting the history of the zoo is now on display depicting the people and animals which helped the attraction survive, thrive and be such an important part of Dudley.
Derek added: "We have 200 staff and the amount of people in Dudley who have worked here or know someone who works here means we are a big part of Dudley.
"We have got some exciting changes happening, as well as ensuring our 20 listed buildings on site are looked after, we are about to do some much needed structural work on the castle itself."
On Saturday, animals were given special presents to open, people turned out in 1930s clothes, a clown provided entertainment and there was a bird of prey in the castle grounds.
Lucy Thorpe, who dressed up in 1930s clothes to celebrate the zoo's birthday, said: "I work at the Black Country Living Museum so wanted to show my support for the zoo's birthday and gave me a chance to wear these clothes which people seem to like."