"After giving it much consideration, I have decided that it is time to retire and move on to another phase of my life," she wrote on social media.
While on the board, Miller at times stirred controversy by clashing with her fellow supervisors, county staff, the media and the business community over county spending, public-records laws, economic development and other issues.
Her announcement came as a surprise to many, considering she had already filed to run for a third term.
Longtime supporter and personal friend Marana Mayor Ed Honea said Miller's announcement has him concerned for the citizens of Marana, much of which is part of District 1. He worries Miller's replacement wouldn't oppose a countywide sales tax, which County Administrator Chuck Huckelberry has long urged the Board of Supervisors to adopt in order to reduce property taxes.
"I am really concerned about the county half-cent sales tax because if it gets a 5-0 vote from the Board, they could do it without going to voters," he said. "Ally and I have been political strategist partners for many years and we share in our ideologies. We're both conservative individuals. She is doing a phenomenal job and I think she always has citizens of her district foremost in her thoughts."
Since Miller's announcement, Oro Valley councilwoman Rhonda Piña (R) has filed to put her name in the race for the heavily Republican District 1.
"I'm a native of Pima County and have an interest in ensuring the county as a whole is a good place to live," she said. "The other thing that's important is I believe I bring business and political experience that can address district and county needs."
Piña was elected to the Oro Valley council in 2016.
She was surprised by the news that Miller would not be running, but praised Miller for her time on the board.
"I have to commend her for her public service work," she said. "Being an elected official isn't easy but it's very rewarding and I would just commend her for her service."
Piña already has competition from two Democrats, both with a background in education, who filed to run against Miller back in September: Jeff Farrell and Rex Scott.
Farrell is a substitute teacher and felt compelled to run for Miller's seat after her comments on Facebook that she was "proud to be white" after clashes between white nationalists and counter-protestors in Charlottesville in 2017 left a counter-protester dead.
"She doesn't have to answer for her incompetence or racism," he said. "I was appalled and disgusted (by her comments) and there's no reason a public official should cultivate a climate that normalizes that behavior."
Farrell is interested in prioritizing road repair, new technologies, bike tourism and would like to see board meetings moved to a time where more community members could attend. He also said he would like to make himself available to constituants to truly learn the needs of District 1.
Miller's announcement is a bittersweet one for Farrell.
"I really wanted to go against her and take a stand," he said. "It's now no incumbent so my chances are better and I'm excited and sad at the same time."
Scott has nearly three decades of experience within public education, most recently serving as principal of Tortolita Middle School.
He said running for supervisor is just another way for him to continue to serve the community after his retirement in July.
Scott wants to improve perception and satisfaction with the county government, as well as make the community a better place through infrastructure and economic development.
"When you are serving a community on behalf of its children, you're learning the needs of the community through the eyes of those kids," he said. "I know if we want kids to stay here when they graduate from high school, there needs to be an economic climate that creates the kind of jobs that make you stay and there needs to be the infrastructure that makes this a quality place to live."
Scott said he is a big believer in dialogue, and if elected would utilize collaboration when making decisions.
"Having been a school leader as long as I have, one thing I've learned is there's nothing good or sustainable that happens in a complex organization like county government or a school that's the result of one person's work or one person's decisions," he said. "I try and get all the facts, weigh them, talk with everyone before I make a decision because when you're in the process of deciding on something that is also a learning experience."
He said he was as surprised as anybody else to hear Miller withdraw from the race, and that he had recently introduced himself to her at a supervisors meeting as a candidate.
"It was a very jocular, light-hearted moment and certainly the takeaway I had from that one and only conversation I've had with her is that she was in the race," Scott said.
Republican and former state lawmaker Vic Williams has also expressed interest in filing to run for District 1.
Williams was elected to the Arizona House of Representatives in 2008 and again in 2010, but lost a Republican primary for Board of Supervisors to Miller in 2012.
"I've always been interested in local government and if you look at my background, I have years of service in Pima County with my local community and the state legislature," he said. "I'm prepared and am looking forward to serve."
Good governance, economic development and bringing jobs and industry to Tucson are some of his top priorities for District 1.
He is committed to remaining visible in the community and hearing the needs of constituents.
Williams hopes that whoever takes the District 1 seat will hold true to Miller's work in bringing awareness to inefficiencies within the government.
"I think Ally did a good job in the northwest and served the constituents well," he said. "She's the most well known supervisor in Pima County."
Along with Williams, former state lawmaker Ethan Orr has also expressed interest in the District 1 race.
Orr said he has received a number of phone calls asking him to run, but he is also interested in running for the Pima Community College board.
Miller will complete the rest of her term, ending on Dec. 31, 2020. The general election is on Nov. 3, 2020 and a primary election is in August 2020.