The longest-running residential tax dispute in Massachusetts history, over the $51 million valuation of a West Tisbury property owned by William W. Graham, has finally ended with a win for the town assessors.

The last chapter in the long legal story closed on Friday with a unanimous decision by the Massachusetts Court of Appeals upholding a 2006 decision by the Massachusetts Appellate Tax Board against Mr. Graham.

Although he could potentially battle on to the state Supreme Court, Mr. Graham has decided not to, and has also dropped other pending actions against the town.

“It seems to me that this is the end of the line,” a deflated Mr. Graham said on Wednesday from his office in Los Angeles. “I’m going to accept the court’s decision and move on.”

The decision was welcomed by the West Tisbury board of assessors and also by their attorney Ellen Hutchinson, who represented them throughout.

“It was, and I think still is, the longest running residential tax case in state history,” Ms. Hutchinson said. “We went 36 days at trial, and then did two views, which totaled five days. We had over 450 exhibits.

“Of course, there were very significant dollar values — over $50 million — involved.

“And it was significant, frankly, in the tone and tenor that it took, in terms of accusing the assessors of fraud. It became personal.

“I’m thrilled to death. I think the assessors, in particular Jo-Ann Resendes, the chief assessor, now the assessor at Edgartown, feel vindicated.

The appeals court decision comprehensively endorsed the state tax board decision and systematically dismissed all the arguments put on Mr. Graham’s behalf.

“On our view of the record,” the decision says, “we are satisfied that the board’s findings were based on substantial evidence and that its reasons for rejecting much of Mr. Graham’s proof were well founded.”

The decision was issued under a little-known rule of the appeals court, rule 1:28, which means the decision is not circulated among other appeals court judges for review and is limited to the three-member panel that reviewed the case. The case may be cited but is not considered as binding precedent.

The saga had its beginning back in the property boom of the late 1990s. Within weeks of each other in 1999, two waterfront properties in the same neighborhood as Mr. Graham’s off Lambert’s Cove, sold for prices in excess of $10 million.

The first sale in July involved a parcel known as the Roberts parcel, of 81.4 acres, which went for $12 million. The following month saw a second sale, referred to in the court as the Ziff sale, in which 12 acres sold for $10.4 million.

When in 2002, the town was completing its triennial reassessment of land tax values, these sales caused assessors to establish a new “neighborhood” for land tax purposes, neighborhood 200, including those two properties as well as Mr. Graham’s 235 acres at Mohu, on seven lots.

Under the complex formula by which values are determined, a base rate is set, which is then ratcheted up according to various criteria. The so-called “multiplier” applied to the new neighborhood was set at 9.5, which resulted in the value placed on Mr. Graham’s land almost tripling.

The legalities were arcane, but the essence of Mr. Graham’s case was that the assessors based their new numbers on a single sale, the Ziff sale, then manipulated the criteria to make the Roberts sale fit them, and then discriminated against him by applying the formula inappropriately to his land even as they excluded other properties from the new neighborhood.

The assessors valued his properties at more than $51 million; Mr. Graham argued their real value was about $20.24 million. His tax bills for fiscal years 2003 and 2004 totaled more than $528,000; he argued they should be reduced by around $300,000.

But the state tax board decision made no changes to the assessments of five lots, and only minor abatements on the other two, lowering the total $51 million assessment by $520,000, meaning the town had to reimburse him $5,460.

Mr. Graham did have a win, however, in one other regard. The tax board accepted that during the 2002 townwide revaluation, assessors had wrongly altered a property record card for the Roberts parcel.

Prior to the 2002 revaluation, the card recorded the property as having water views. This was changed to say “no view.” West Tisbury’s principal assessor, Jo-Ann Resendes and an employee of the town’s contracted valuation company, June Perry of Vision Appraisal, testified they had visited the site and found the view obscured by trees.

In fact, the appellate board found extensive views, something it deemed to have diminished the assessors’ credibility, but not to have undermined the overall validity of the appraisal.

Before the appeals court, Boston counsel for Mr. Graham, John Stevens, made much of the assessor’s inaccurate testimony, calling it a lie.

“The assessments challenged in this appeal were based upon errors of law and justified by a factual statement the Appellate Tax Board found to be a lie,” he wrote in a brief to the court.

But in this week’s decision, the appeals court dismissed this, as it dismissed various other strands of the Graham case, as having failed to meet the burden of proof.

The appeals court found that the assessors’ designation of the new neighborhood 200 was warranted, considering the two 1999 sales in excess of $10 million.

It did note the establishment of neighborhood 200 depended on several variables and “to a certain extent a subjective judgment to be made by the assessors,” but accepted that the “no view” notation did not affect the calculation of the tax.

And the suggestion that the assessors had “engaged in an intentional scheme of disproportionate assessment” did not meet the burden of proof.

“Graham failed to carry his burden of showing that the assessors’ methodology resulted in overvaluation of his property,” the decision says.

While awaiting last week’s result, Mr. Graham filed appeals against his tax assessments for every year up to the present, but now will withdraw those appeals.

He maintained his challenge had never been about money so much as principle.

“I brought the case because I thought there were inconsistencies in the way that properties are assessed. I still think there are,” he said, adding:

“And if you look at the number of challenges from taxpayers in the most recent revaluation, you’ll see there are a lot of West Tisbury taxpayers who have come to agree with me.”

However, having spent many tens, if not hundreds of thousands of dollars (he declined to reveal how much), Mr. Graham said it was now a matter of “being sensible,” and calling it quits.

“But having said that,” he continued, “I’ll be paying close attention, and I think a lot of other West Tisbury taxpayers will be paying attention, to the next revaluations.”

He believed that process would start in 2010.

“Maybe we’ve all learned to be vigilant and be more involved as the process goes along,” Mr. Graham said.