Do I need a civil marriage as well as a nikah?

Yes, in practice you should. Islamically, a nikah with its conditions met is a complete, valid marriage. But in many Western countries a religious ceremony alone isn't recognized by the state, so a civil marriage (or a nikah that's also legally registered) is what protects you, your spouse and your children.

The nikah is valid before Allah

A nikah needs mutual consent, a mahr, witnesses and, in most schools, the bride's wali. The Quran calls marriage "a solemn covenant" (Quran 4:21), and once those conditions are met you're married in the eyes of Islam. You don't need a government stamp for the marriage to be halal. See Do I need a wali to get married as a Muslim convert woman?

The UK: a nikah alone isn't a legal marriage

In England and Wales, a nikah held at home or in most mosques isn't legally recognized. Unless it takes place at a building registered for marriages with an authorized person present, the law treats the couple as unmarried. You'd need a separate civil ceremony at a register office or approved venue, either before or after the nikah. Many imams in the UK now ask couples to do this, and some won't perform a nikah without it. Scotland and Northern Ireland have their own rules, so check locally.

The US: it depends on the state

In the US, marriage is regulated by each state. Generally you get a marriage license from the county first. In many states, an imam can then solemnize the marriage, and once the signed license is returned, your nikah is also your legal marriage. The rules on who can officiate, waiting periods and witnesses differ, so check your county clerk's website. Without a license, a nikah usually has no legal effect.

Why the legal side matters

Protection for the wife: without a legal marriage, she may have no claim to the home, maintenance or pension if the marriage ends or her husband dies.
Inheritance: an unmarried partner often inherits nothing without a will.
Children: legal marriage makes parentage, custody and paperwork simpler.
Immigration, tax and hospital visits: these rely on a legal marriage certificate.

Scholars and fiqh councils in the West strongly encourage registering. Islam cares about protecting rights, especially women's, and an unregistered marriage can leave a wife badly exposed. Registering is also part of following the law where you live. See Do I have to follow the laws of my country as a Muslim?

Already married civilly?

If you married civilly before Islam and both of you are now Muslim, or your spouse is Christian or Jewish, your marriage is usually still valid and a new nikah is optional. See Is my church wedding still valid after becoming Muslim?

Key points

A proper nikah is a valid Islamic marriage on its own.
In the UK, a nikah alone usually isn't legally recognized.
In the US, an imam can often perform a legal marriage with a state license.
Registering protects the wife, children and inheritance rights.

Tip: Book the civil ceremony or license as part of your wedding plans, not as an afterthought. Ask the imam early whether he's authorized to register marriages where you live.