It is clear that he was aligned to Charles's policy in 1672 and supported the Declaration of Indulgence especially in so far as it affected dissenters (and potentially extending this to Catholics, but this was always an ambiguous point)[clarification needed].

He paid close attention to Irish interests, and took immense pains to understand the constitution and the political necessities of the country, appointing men of real merit to office, and maintaining exceptional independence from solicitation and influence.

[2][3] He immediately joined the country party and the opposition to Lord Danby's government, and on the latter's fall in 1679 was appointed a commissioner of the treasury, and the same year a member of Sir William Temple's new-modelled Council.

He now, in 1680, joined Shaftesbury's party and supported the Exclusion Bill, and on its rejection by the Lords carried a motion for an association to execute the scheme of expedients promoted by Halifax.

[2][3] Essex took no part in the wilder schemes of the party, but after the discovery of the Rye House Plot in June 1683, and the capture of the leaders, he was arrested at Cassiobury and imprisoned in the Tower.

Capell also engaged the services of the leading wood carver of the day, Grinling Gibbons, and of the painter, Antonio Verrio, to create a sumptuous interior.

[6] Arthur Capell married Lady Elizabeth Percy, daughter of Algernon Percy, 10th Earl of Northumberland and Lady Anne Cecil, by whom he had children: The Earl of Essex died in the Tower of London on 13 July 1683, having been convicted of participation in the Rye House Plot against the King and his brother, and was said to have been discovered in his chamber with his throat cut[2] whilst a prisoner awaiting execution for treason.

[7] According to Britton, the Earl of Essex's death, by suicide, was controversial: Lawrence Braddon., Gent of the Middle Temple states himself 'upwards of 5 years persecuted or imprisoned for endeavouring to discover this murther the third day after the same was committed' [8] The Dictionary of National Biography entry for Braddon says:- "When the Earl of Essex died in the Tower in 1683, Braddon adopted the belief that he had been murdered, and worked actively to collect sufficient evidence to prove the murder.

For a time he was let out on bail, but on 7 Feb. 1683–4 he was tried with Mr. Hugh Speke at the king's bench on the accusation of conspiring to spread the belief that the Earl of Essex was murdered by some persons about him, and of endeavouring to suborn witnesses to testify the same.

His death was attributed, quite groundlessly, to Charles and James, and the evidence points clearly if not conclusively to suicide, his motive being possibly to prevent an attainder and preserve his estate for his family.

He went into a small closet," where his servant afterwards found him "dead and wallowing in blood"... the assumption being that the reason he "cutt his own throat with a knife" was because of his knowledge of the Rye House Plot.

John Evelyn describes him as "a sober, wise, judicious and pondering person, not illiterate beyond the rule of most noblemen in this age, very well versed in English history and affairs, industrious, frugal, methodical and every way accomplished"; and declares he was much deplored, few believing he had ever harboured any seditious designs.